SB 608 

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^py i SCAB DISEASE OF APPLES 



A THESIS 

Presented to the Faculty op the Graduate School 
of Cornell University for the Degree of 

DOCTOR OF PHILOSOPHY 



BY 



ERRETT WALLACE 



(Reprinted from Cornell Agricultural Experiment Station Bulletin 335, September, 1913] 



SCAB DISEASE OF APPLES 



A THESIS 

Presented to the Faculty of the Graduate School 
of Cornell University for the Degree of 

DOCTOR OF PHILOSOPHY 



BY 

ERRETT WALLACE 
it 



[Reprinted from Cornell Agricultural Experiment Station Bulletin 335, September, 1913] 



CONTENTS 

PAGE 

The host 545 

The apple in the United States 54.5 

The apple in New York 546 

The disease 546 

Names applied 546 

History 547 

Symptoms 547 

On the leaves 547 

On the fruit 548 

On the twigs 548 

Importance 548 

General estimates of loss 548 

Estimates of loss in New York 549 

Nature of the loss 551 

Loss of fruit set due to the disease 552 

Pink rot following scab 553 

Etiology 554 

Morphology 554 

Nomenclature 556 

Relationships and host plants 557 

Life history 557 

Perithecial stage 557 

Development of perithecia 558 

Time of maturity of ascospores 559 

Discharge of ascospores 560 

Cultural characters of the fungus 564 

Artificial inoculations 565 

Method of infection 567 

Time of infection 568 

Place of primary infection 572 

Late infection and scab development in storage 574 

How the fungus passes the winter 576 

Vitality of conidia 576 

Hibernation of conidia 576 

Persistence of stroma on twigs 577 

Formation of appressoria 578 

Summary 578 

Varietal susceptibility 579 

Control 582 

Sanitary measures 582 

Selection of resistant varieties 584 

Spraying. . 584 

Fungicides 584 

Bordeaux mixture 585 

Dust sprays 586 

Lime-sulfur preparations 587 

Lime-sulfur solution 587 

Scott lime-sulfur 588 

Waite's modification 588 

When to spray 588 

First application 588 

Later applications 589 

Dormant spraying 589 

Spraying fallen leaves ' 590 

Summary 590 

Effect of continued spraying • 591 

Bibliography 593 

543 



SCAB DISEASE OF APPLES* 

Errett Wallace 

(Received for publication June I, IQI3) 

THE HOST 

Consideration of the origin, evolution, distribution, and methods of 
care and cultivation of the plants affected by a specific disease is often 
of assistance in reaching conclusions as to the origin and history of the 
disease, its possible distribution, and its economic importance. In the 
case of the apple these facts are rather generally known or are readily 
obtained from various horticultural books. To residents of New York 
State this information is especially accessible in the excellent work by 
Beach (1905) entitled " The Apples of New York." It is therefore not 
necessary, in this bulletin, to enter into an extended discussion of these 
features further than to indicate in a general way the distribution and 
importance of the apple industry. 

THE APPLE IN THE UNITED STATES 

Throughout the United States the apple is more generally cultivated 
than is any other fruit. The range of latitude in which it can be 
grown and the diversity of soil suitable for apple culture makes possible 
this wide distribution ; while the universal demand for the fruit as a staple 
article of diet offers a special inducement for its production. 

The census report for 19 10 shows that apples are being grown in every 
state and territory in the Union except Alaska. In some of these localities, 
however, such small quantities are produced that the industry cannot 
be considered to be of commercial importance. Among these may be 
mentioned Florida, Louisiana, Wyoming, Arizona, Nevada, and North 
Dakota, in each of which States less than 100,000 bushels of apples were 
produced in 1909. 

The accompanying table, compiled from the Thirteenth Census Report, 
shows the production of the leading apple-growing States in 1909 and in 
1899, together with the total production in the United States for those 
years: 

* Also presented to the Faculty of the Graduate School of Cornell University, June, 191 1, as a major 
thesis in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 

(1905) Beach, S. A. The apples of New York. Vol. I. ' New York (Geneva) Agr. Exp. Sta. Rept. 
1903:2: 1-409. 

545 



546 Bulletin 335 

TABLE 1. Production of Apples in 1909 and in 1899 



1909 



Product inn 
(bushels) 



Value 






iction 
(bushels) 



United States. 
New York .... 

Michigan 

Pennsylvania. . 

Missouri 

Kentucky .... 

Iowa 

California. . . . 

Virginia 

North Carolina 

Ohio 

Tennessee. . . . 
West Virginia. 
Maine 



147,522,318 

25,409,324 

12 ,352 ,296 

1 1 ,04s 1430 

9.968.977 

7,368,499 

6,746,668 

6,335,073 
6, 103,941 

4.775.693 
1,663,752 

4 ■ 640 . 444 
4.225,163 
3,636,181 



,83,231,492 

13,343,028 

5,969,080 

5.557,6i6 

4.885.544 
3,066,776 

3 - 550 , 729 
2,901 ,662 
3,129,832 
2,014,670 
2,970,851 

2,172,475 
2,461 ,074 
2,121 ,816 



I75.397.6oo 

24, 1 1 1 , -57 

8,931,569 

24,060,65] 

6,496,436 
6,053.717 
3, 129,862 
3,488,208 

9,835.082 

4,662 ,751 

20,617 ,480 

5,387,775 

7.405.743 
1. 421. 773 



THE APPLE IN NEW YORK 

In New York State the apple has assumed an important place in recent 
years. It is the basis on which rests an industry that has made independent 
thousands of farmers throughout the State and has given to many com- 
munities a general air of prosperity and thrift, for which the rural sections 
of this State, as a rule, are noted. In western New York, particularly, 
the soil and the climate are so well adapted to apple-growing that the 
industry has assumed notably large proportions. In this State the two 
great commercial varieties, Baldwin and Rhode Island Greening, nourish; 
however, although taking the lead, these are by no means the only 
varieties of importance. 

The Thirteenth Census shows that New York produced in 1909 more 
than twenty-five million bushels of apples, worth over thirteen million 
dollars. This production is more than twice that of any other State. 
The production of all orchard fruits in the State amounted to 29,456,291 
bushels, with a value of $17,988,894. It is therefore evident that the 
apple is of much greater commercial importance in New York State than 
are all other orchard fruits combined. 

THE DISEASE 

NAMES APPLIED 

The disease, which is known as scab, black spot, scurf, or the fungus, 
and which has in some cases been called rust, occurs on the leaves and the 



Scab Disease of Apples 547 

fruit, and occasionally has been found on the twigs, of the apple. The 
name "scab" is used almost exclusively in the United States and is the 
name that will be used in this account. 

HISTORY 

The scab disease apparently exists in every country where apples are 
grown. It was reported early in the nineteenth century by Fries (18 19) 
from Sweden, and some years later by Wallroth (1833) from Germany. 
The first authentic record from America is by Schweinitz (1834), who 
reports scab on Newton Pippins in New York and Pennsylvania. The 
disease was first noticed in England in 1845, according to Berkeley (1855), 
and in Australia in 1862, according to McAlpine (1902). Its introduction 
into Australia was attributed to a Seckel pear imported from America; 
this is doubtful, however, since the scab disease of the pear has since 
been shown to be entirely distinct from that of the apple. 

SYMPTOMS 

On the leaves 

The scab is likely to appear earliest on the lower side of the leaves. 
The diseased area usually appears first as an olive discoloration slightly 
darker than the normal surface of the leaf. The color deepens with age 
until dark brown or black is reached, the spot having a more or less velvet- 
like appearance. As noted by Aderhold (1896), on the lower side of the 
leaf there is a tendency for the lesion to extend along the veins and the 
midrib and to diffuse irregularly and indefinitely into the healthy area; 
whereas on the upper surface of the leaf the lesion appears first as a slight 
olive-green discoloration, of a lighter shade of green than the healthy 
surface of the leaf but dull and somewhat velvety. The natural luster 
characteristic of the upper surface of the leaf is destroyed. The spots 
may be few and scattered; or they may be so numerous as to coalesce, 
coating almost the entire surface. The diseased areas may be distinctly 
bordered or they may spread out irregularly and indefinitely into the 
healthy part of the leaf. In Plate I, Plate II, Fig. 1, and Plate III, Fig. 
2, are shown various types of infestation, on both the upper and the lower 
surfaces of the leaves. 

Later the scab spots become darker, changing to brown and finally, 
in some cas es, to nearly black. In some cases the natural form of the leaf 

(1819) Fries, Elias. Spilocaea Pomi Fr. Nov. fl. Suec. 5:70. 

(1833) Wallroth, F. G. Cladosporium dendriticum W. Fl. crypt. Germ. 2:4:169. 

(1834) .Schweinitz, L. D. de. Spilocsea fructigena aut Pomj Lk. Syn. F. N. A., p. '207. 
(1855) Berkeley, M. J. Why do pears and apples crack ? Gard. chron. 1855:724.' 
(1896) Aderhold, Rudolf. Die Fusicladien unserer obstbaume. Landw. jahrb. 25:881 

(1902) McAlpine, D. The fungus causing black spot of the apple and pear. Victoria Agr. Dept. Journ. 
1 : 7°3 _ 7o8. 



548 Bulletin 335 

is not destroyed; in other eases some distortion results. Very often the 
diseased surface protrudes, forming a convex surface with a corresponding 
concavity on the opposite side of the leaf. In time the tissue under many 
of the diseased spots is killed outright, forming dead areas, which often 
crack as shrinkage occurs. This condition is most common near the 
close of the season. In extreme cases considerable defoliation may result 
(Plate II, Fig. 2). 

On the fruit 

The lesion on the fruit usually appears from the first as a darker-colored 
spot than is produced by the disease on the leaves. Sometimes the spots 
are almost black when first visible; or they may be dark olive, changing 
to reddish brown or black. The spots are usually very small at first and 
they enlarge more slowly than do those on the leaves. They are more 
sharply bordered on the fruit than on the leaves (Plates IV and V). 

As the scab spot on the fruit grows older its appearance changes 
markedly. The central and older part becomes bare, brown, and cork} , 
while the margin is black. A more or less whitish band, due to the 
loosened cuticle, may surround the black margin. (Plate VI, Fig. 2). Some- 
times the scab spots may enlarge so as to cover rather large areas and 
cracking of the fruit results, due to excessive loss of moisture content from 
the underlying unprotected tissues (Plate III, Fig. 1). 

Scab spots resulting from late autumn infection differ somewhat in 
appearance from those developing early while the fruit is very young. 
The spots as they first appear are more dense and black. Often they 
will have enlarged considerably before the cuticle is ruptured. They 
seldom reach the stage described above, in which the center becomes 
bare and brown (Plate VI, Fig. 1). 

On the twigs 

The disease rarely occurs on the twigs, at least in many localities. 
Affected twigs have, in general, a scurfy appearance. The bark becomes 
blistered and later ruptured in places, presenting an appearance similar 
to scabby pear twigs, which are very common. The occurrence of twig 
infection is discussed at greater length elsewhere in this bulletin. 

IMPORTANCE 

General estimates of loss 
McAlpine (1902) estimated the annual average loss due to apple scab 
in. Victoria, Australia, at £40,000 (approximately $194,000), which is 

(1902) McAlpine, D. The fungus causing black spot of the apple and pear. Victoria Agr. Dept. Journ. 
1:705. 




Plate II. Fig. i. — A severe infestation of scab artificially produced by inocula- 
tion of protected foliage with ascospores of Venturia inccqnalis. Photograph 
made twenty days after inoculation. Natural size 

Fig. 2. — Badly infested leaves as they appear late in the season. Photograph 
made on August ij, iqoS. Natural size 



^-..--.- . -. \ 





«sr* 






2 



Plate III. Fig. i— Severe infestation of scab. The fruit is cracked in places, 
while the old spots appear corky. Many of the spots are small, the result of late 
infection. Photograph made on October 14. iqoS. Natural size 

Fig. 2.— Scab lesions on upper surface of leaf only, producing a cup-like effect 
beneath. A common occurrence in cases of severe infestation. Photograph 
made by Fisher, August 7, IQ12. Natural size 




Plate IV. — Cluster of badly scabbed apples showing typical lesions. Photograph 
made by Whetzel, July 31, igo6. Natural size 




Plvte V Fig. i.- Successive stages in the development of ^f ^/ #*£ 
Zicrographofthelesionontheupperleftapple t sshown i n Plate VIII, Fig.6. Photo 

FIG OBOWlwg gra ph iiaL by Fisher, August 7, 19"- Natural stze 













• 


• » 


» * 


' 





4f 




" i 


1 




Pi. An-; VI. Fig. i. — Scab lesions on fruit. Late, secondary infections 

are becoming prominent. Photograph made in the autumn of iqoq. 

Natural size 
Fig. 2. — Showing the action of the scab fungus in lifting the cuticle of 

the apt>le, thus allowing evaporation. Photograph made by Whetzel, 

July ji, iqo6. Enlarged twice 




Plate VII. Fig. i. — Pear scab lesions on leaves and pedicels. Pedicel infection 
is equally common on apple, but cannot lie shown so well in photograph because of 
dense growth of hairs. In the orchard from which this twig was taken, practically 
no fruit set because of the attack on the pedicels. Photograph made on June i, 
IQOQ. Natural size 

FlG. 2. — Fruit pedicels and foliage severely infected at an early stage -with the scab 
fungus. The apple in >he upper right corner no! so severely attacked. The fruit 
will never mature. Photograph made on August 15, iqo8. Somewhat reduced 



Scab Disease of Apples 549 

equivalent to $48.50 an acre. Stevens and Sherman (1903) state that one 
grower reports an increase of S1000 in value of his crop due to spraying, 

at an outlay of $125 to Si 50; and that in the State of Illinois apple 
scab is estimated to have caused $6, 000, 000 damage in one year, or sixty 
per cent of the total loss through all enemies. Marlatt and Orton (1906) 
state that the loss from scab amounts to many million dollars each year. 
Scott and Quaintance (1907) estimate that scab often affects fifty to 
seventy-five per cent of the fruit over wide areas, and not infrequently 
causes total failure of the crop by killing the young fruit when in blossom 
or soon after. Gossard (1908) estimates a net profit of $3 to $7 for each 
tree, due to spraying for control of insects and disease, at a cost of 30 to 
50 cents for each tree. 

Estimates of loss in New York 

Some interesting figures in regard to the importance of scab disease, 
as shown by the gain due to spraying apple orchards, are to be found 
in the reports on orchard surveys of Orleans and Niagara counties. Warren 
(1905) reports an average gain of S47 an acre, in 1904, in orchards sprayed 
three times over those not sprayed. This is included in a study of 564 
orchards containing 48S1 acres. Since it was thought that other forms of 
neglect might be correlated with that of omitting the spray, another 
count was made including only those orchards that were well cared for 
in other ways. The difference in this ease was $81 an acie in favor of 
orchards sprayed three times. Evidently, then, the profit from spraying 
well-cared-for orchards is greater than that from spraying those otherwise 
neglected. This is what should be expected, since trees properly cared 
for are better able to produce a larger quantity of fruit than those not 
well cared for. The cost of spraying was estimated at $6.77 an acre, 
leaving an average net gain cf $40.23 including all orchards and of $74.23 
including only orchards otherwise well cared for. 

Here, of course, it is impossible to separate the percentage of gain due 
to the control of scab from that due to the control of the codling moth, 
since the disease and the insect are controlled by a combination spray. 
An attempt was made to correlate the percentage of scab with the vield 
of fruit and the income for each acre. It was found that orchards having 
no scab to 5 per cent of scab gave an average income of $143 an acre, 
with a yield of 382 bushels; while those having 76 to 100 per cent of scab 

(1903) Stevens, F. L.,and Sherman, Jr., Franklin. Insert and fungus enemies of the apple, pear, and 
quince, with methods of treatment. Xorth Carolina Agr. Exp. Sta. Bui. 183:66. 

(1905) Warren, G. F. An apple orchard survey of Orleans county. Cornell Univ. Agr. Exp. Sta. Bui. 
229:478. 

(1906) Marlatt, C. L., and Orton, W. A. The control of the codling moth and apple scab. U. S. Agr. 
Dept. Farmers' bul. 247:12. 

(1907) Scott, W. M.. and Quaintance. A. L. Spraying for apnle diseases a'nd the codling moth in the 
Ozarks. U. S. Agr. Dept. Farmers' bul. 283:20. 

(1908) Gossard, H. A. Spraying apples. Ohio Agr. Exp. Sta. Bul. 191:103-125. 



55o Bulletin 335 

gave an average income of only $88 an acre, with a yield of 248 bushels. 
This shows an apparent loss of $55 an aere where scab was abundant. It 
is probable, however, that the scabby orchards were unsprayed, and there- 
fore suffered from the codling moth also. It is seen likewise from the 
figures given that these pests reduced the quantity of the yield 134 bushels 
an acre. 

Cummings (1909) made similar estimates in his survey of Niagara 
county. Here unsprayed orchards gave an average yield of 261 bushels 
with an average income of $45 an acre, while three sprayings resulted 
in a yield of 577 bushels and an income of Si 71 per aere; a gain in yield 
of 316 bushels and in income of Si 26 an acre. 

Cummings finds that there are in Niagara county about 24,190 acres 
in apples, and on this acreage about one fifth of the orchards are unsprayed. 
From this it may be estimated that the loss in the 4838 acres of unsprayed 
orchards in Niagara county would aggregate $609,588. An estimate 
similarly made from the figures already inferred to by Warren for Orleans 
county shows that the loss suffered by unsprayed orchards in that county 
during' the season of 1904 was about $287,616. 

These are apparently very moderate estimates. Only the difference 
between orchards unsprayed and those sp rayed three times was con- 
sidered. Many orchards were ineffectively sprayed; they also suffered 
much loss and were not included in these estimates. 

Taking Orleans county as a basis from which to work, the writer has 
attempted to estimate roughly what may be the total annual loss due 
to neglect of spraying in New York State and in the United States. As 
shown above, the loss in Orleans county in 1904 was $47 an acre. Accord- 
ing to the census of igoo there were in New York State at that time 
15,054,832 apple trees of bearing age. Warren found that there are, on an 
average, slightly more than forty trees per acre, from which it may be 
estimated that there were about 376,376 acres of apple orchards in the 
State. Cummings found one fifth of the orchards in Niagara county 
unsprayed. Since that is one of the most progressive fruit-growing counties 
in New York, there is no doubt that it would be a conservative estimate 
to say that one fifth of the orchard area of the State is unsprayed, making 
a total of 75,274 acres on which a loss of $47 an acre would be suffered. 
This gives S3.537.S7S as an estimate of the annual loss to this State through 
neglect of spraying. This estimate represents only the loss where no 
spraying is done. Many thousands of acres are ineffectively sprayed, 
and if the loss on these could be estimated it would add much to the 
aggregate. 

(iqoo) Cummings, M. B. ■ Apple orchard survey of Niagara county. Cornell Univ. Agr. Exp. Sta. Bui. 
262: 277-320. 



Scab Disease of Apples 551 

Figuring on a similar basis for lite United States, it is found in the same 
census that there were 201,71)4,764 apple trees on 5,044,869 acres. One 
fifth of this acreage gives [,008,973.8 acres of unsprayed orchard, on which 
the loss at $47 an acre would amount to $47,421,768.60. 

These estimates are based on by far the lower of the figures showing 
the average loss found to exist in the two eases cited above. Accord- 
ing to figures of Cummings for Niagara county, in 1905 there was a gain 
of Si 26 an acre in the income from sprayed over that from unsprayed 
orchards. If this amount is considered as representing the average loss 
through neglect of spraying, the figures given here would be increased very 
materially. The writer has chosen the more conservative estimate, how- 
ever, and believes he is safe in stating that New York alone loses over 
three million dollars annually through this one form of neglect on the part 
of its apple-growers, and that a corresponding loss of over forty-five 
million dollars is suffered by growers throughout the United States. 

It is of course impossible to determine what proportion of this loss is 
due to scab. In the northern States scab is the all-important fungous disease 
of the apple and is the cause of a large proportion of the loss; while in 
some of the more southern States other diseases, such as bitter rot, blotch, 
and the like, outrank scab in importance. While the codling moth is generally 
distributed throughout the United States and is very destructive, a large 
percentage of wormy apples are also scabby and the disease alone would 
be responsible for great loss even if there were no codling moth. 

Nature of the loss 

Ordinarily the reduction in quality of scabby apples is considered to 
be the main cause of loss. This, while it is important, is only one of the 
several factors. It will be shown later that early scab infection, if not 
controlled, in some years almost entirely prevents the setting of fruit; 
it also very materially reduces the size of individual apples, while a single 
lesion retards growth on the affected side and causes unsymmetrical 
development ( Plat eV, Fig. 2). The unmistakable dwarfing effect has been 
apparent in experimental work during two epidemic years of apple scab, 
when it was noticed that apples from unsprayed trees were uniformly 
smaller than those from sprayed trees. Green (1891) determined that 
there was a loss in size of fifty per cent on scabby fruit as compared with 
sound fruit. 

A fourth important factor of loss to be considered is the effect on the 
keeping qualities of the fruit. This effect is indirect, in that the disease 
furnishes a point of entrance for Cephalothecium roseum, the pink-rot 
organism, Penicillium expansion, the brown ripe-rot fungus, Sphaeropsis 
malorum, the black-rot fungus, and other organisms. 

(1891) Green, W. J. The spraying of orchards. Ohio Agr. Exp. Sta. Bui. 4:9: 193-212. 



552 Bulletin 335 

In addition to the immediate effect on the crop of the current year, 
there is doubtless, in case of severe leaf infection, a devitalizing effect on 
the tree as noted by Bailey (1895, pp. 13-14). To some degree this pre- 
vents the formation of fruit buds for the following year and hinders the 
normal wood growth which is the basis for future crops. 

Loss of fruit set due to the disease 

The majority of apple-growers have believed for years that the occur- 
rence of cold rains during the blossoming period is the cause of failure 
of fruit to set. There is ample evidence, however, that the scab disease 
occurs abundantly on the pedicels in certain years — as in 19 10 — and 
causes the blossoms or the young fruit to fall (Plate VII, Figs. 1 and 2, and 
cover-page figure). It is even claimed by Reddick (19 13) that cold rains at 
blossoming time are not a factor in the setting of fruit, but that the scab 
disease is the factor involved. The following instances may be cited as 
showing that destruction of the blossoms and the young fruit by scab 
furnishes an important source of loss, which is often overlooked because 
of the inconspicuous character of the disease at this stage or is attributed 
to various causes such as poor pollination, bad weather, and the like: 

A general failure of the apple crop apparently due to this cause is noted by 
Bailey (1S95) to have occurred during the summer of 1894. In regard to 
the cause of this failure Bailey writes (on page 20 of the bulletin cited) : 
" I have visited over twenty orchards in the western part of the State 
this year in which there were large crops of excellent quality, but all of 
these had been sprayed with paris green or bordeaux mixture, or both, 
all of them were pruned and the land was in ' good heart.' " In general 
the orchards were almost barren in that year, and the smallness of the 
crop was usually in proportion to the degree of neglect to which the 
orchards were subjected. In another place Bailey states (on page 10): 
" But the immediate cause of most of our apple failures of the last few 
years, is undoubtedly the apple scab fungus." Again (on page 18) : " The 
best proof that the apple scab fungus is the immediate cause of the greater 
part of the apple failures of western New York is afforded by the fact 
that thorough spraying with bordeaux mixture is usually followed by a 
great increase in the productiveness of the orchard." While it is not 
stated here just how the disease so greatly decreased the productiveness 
of the orchards, there is no doubt that the decrease was brought about 
principally by the occurrence of scab on the pedicels of the blossoms or 
the young fruit at an early stage, causing them to fall. 

(1895) Bailey, L. H. The recent apple failures of western New York. Cornell Univ. Agr. Exp. Sta. 
Bui. 84:1-34. " 

(1913) Reddick, D. The apple scab situation. West. New York Hort. Soc. Proc. 58:86-90. 



Scab Disease of Apples 553 

Lodeman (1895) cites an instance in which the crop was entirely 
destroyed by this form of attack, as shown by the fact that unsprayed 
trees bore no fruit while there was a large crop on trees properly sprayed. 

Marlatt and Orton (1906) state that " the yield of fruit per tree is 
greatly lessened whenever scab is present: (1) by the premature dropping 
of young apples, due to the attacks of the scab fungus on flowers, stems, 
and fruits soon after the blossoms fall; (2) by the smaller size of the scabby 
apples that mature; and (3) by the loss, just before picking, due to the fact 
that scabby fruit does not cling well to the tree." 

Scott and Quaintance (1907) note that scab not infrequently causes 
total failure of the crop, by killing the young fruit when in blossom or 
soon after. 

Gossard (1909) reports that scab disease caused a large proportion 
of the young fruit to fall from unsprayed Winesap trees almost as soon 
as it had set. 

Taft and Wilken (1909), in their report for 1908, make the fol- 
lowing statement: " If the early spraying was done at the proper time, 
the work of the fungus which attacks the blossom stem and causes the 
blossom to drop was prevented." 

Selby (19 10) states that scab often causes the young fruit to fall, and 
that often this falling or failure of the fruit to set, which is attributed to 
frost injury or poor pollination, is really due to scab. 

Pink rot following scab 

This is another important factor for consideration in connection with 
the causes of loss due to apple scab. The first serious outbreak of pink 
rot apparently occurred during the season of 1902 and was reported almost 
simultaneously by Eustace (1902) of Geneva and by Craig and Van 
Hook (1902) of Cornell University. The fungus causing the disease, 
Cephalothecium roseum Cda., was found to attack the apple almost entirely 
through wounds in the cuticle and in the epidermis caused by scab. The 
fungus had formerly been considered a saprophyte, although Aderhold 
(1899) had reported a case in which it caused rot of pears following scab 
infection. 

(1895) Lodeman, E. O. The spraying of orchards. Cornell Univ. Agr. Exp. Sta. Bui. 86:119. 
_ (1899) Aderhold. Rudolf. Arbeiten der botanischen abteilung der Versuchsstation des Kgl. pomolo- 
gischen Instituts zu Proskau. Centbl. bakt. 2:5:522. 

(1902) Eustace, H. J. A destructive apple rot following scab. New York (Geneva) Agr. Exp. Sta. 
Bui. 227:367-389. 

(1902) Craig. John, and Van Hook, J. M. Pink rot, an attendant of apple scab. Cornell Univ. Agr. 
Exp. Sta. Bui. 207:157-171. 

(1906) Marlatt, C L., and Orton, W. A. The control of the codling moth and apple scab. U. S. Agr. 
Dept. Farmers' bul. 247: 12. 

(1907) Scott, W. M., and Quaintance, A. L. Spraying for apple diseases and the codling moth in the 
Ozarks. U. S. Agr. Dept. Farmers' bul. 283:20. 

(1909) Gossard, H. A. Apple spraying in 1908. Ohio Agr. Exp. Sta. Circ. 95:4. 

(1909) Taft, L. R.. and Wilken, F. A. Annual report of the South Haven sub-station for 1908. Michigan 
Agr. Exp. Sta. Spec. bul. 48:16. 

(1910) Selby, A. D. A brief handbook of the diseases of cultivated plants in Ohio. Ohio Agr. Exp. Sta. 
Bul. 214:371. 



554 



Bulletin 335 



According to the three authors first named above, apple scab had been 
very common during the summer of 1902. Late in the season, in August 
and September while the fruit was yet. on the trees, it was observed, as 
stated by Eustace, that on some of the scab spots there appeared a white 
or pinkish, mildew-like growth. A little later this growth produced a 
brown, sunken, bitter rotten spot. On very scabby apples these spots 
soon coalesced and the fruit became a mass of decay. 

Some rot develops in the fruit while it is on the trees, as stated above, 

but the greater de- 
struction occurs soon 
after the fruit has 
been stored or while 
it is in piles on the 
ground. The occur- 
rence of this disease 
has been common to 
a greater or less 
degree since 1902. 
During the season 
of 1 9 10 considerable 
pink rot developed 
in many sections of 
the State where scab 
was not kept well 
under control. 




Fig. [82. Conidial stage of Venturia incequalis. The section 
through a scab spot shows the fungus stroma located beneath 
tlii- cuticle and in the epidermal cells, conidi-ophores, and 
the development of conidia. The growth of the fungus, 
especially the development of conidia, lifts the cuticle. A 
w of the conidiophores show the scars where, spores were 
developed earlier in the season. (See also Plate VI, Fig. 
2.) Camera lucida drawing 



KTIOI.OGY 



MOI " . 

The apple-scab 



disease is caused by 
the fungous pathogen Venturia incequalis. The mycelium of the fungus 
is in some cases hyaline when young, but it soon becomes tinted and 
varies from olivaceous to reddish brown. Tt is septate, branches very 
irregularly, and is subject to modifications as influenced by environment, 
varying particularly in the different stages of the life cycle of the fungus. 
The hyphae vary from 3 to 5 ^ in diameter in the living host, but in the 
dead leaves they may be as much as twice this size (Plates IX and X). 
In the living host the mycelium will be found, in the earl} stages on 
leaves, confined to a region between the cuticle and the epidermal cells; 
on the fruit the epidermal cells are attacked and usually destroyed. 
Previous to the formation of conidia this subcuticular mycelium divides 



Scab Diskasi: of Apples 



555 



into one or more layers of densely packed, rounded, and at first hyaline 
cells (stroma), from which the conidiophores arc produced (Fig. 182). 
The outer layers later take on a brownish tint. On the leaves this stroma 
may consist of only one layer of cells, hut more than one layer is likely 




Fig. 183. — Mature perithecium of Venturia incequalis. The section 
shows the method of formation of asci, mature ascospores, the method 
of ejection of ascospores, and the bristles that sometimes occur about 
the ostiole. Outlined with a camera lucida 

to occur over the veins or the firmer parts o\ the leaves. On the fruit 
the number of layers is much greater, due apparently to thefact that the 
thicker cuticle oilers a greater resistance which is not overcome until 
the time for the formation of more cells has elapsed. This resistance 
is apparently overcome partly by means of a solvent action of the fungus 



556 Bulletin 335 

(as shown in Fig. 182, where the cuticle is found to have been eaten away 
from beneath, directly over the subcuticular mycelium), and partly by 
the pressure exerted which finally 1 tries the cuticle loose. The conidio- 
phores that arise from these cells are reddish brown in color and while 
young are somewhat ovate in form. They have an inner hyaline wall 
which protrudes from the apex to form the spore. At first the spores are 
hyalind and are rounded in a sac-like manner at the free end, but later 
they become reddish olive-colored and are mostly lanceolate but some- 
what irregularly so and variable in form. They are mostly unicellular, 
but in the later stages a septum is often formed. They are rather variable 
in size, measuring 12 to 22 ju in length by 6 to 9 p. in breadth. The form 
and general appearance of the conidiophore changes with its age and the 
number of conidia that it has produced. The older conidiophores assume 
a more or less irregular form, showing a distinct knee and a change in 
the axis of growth at the point where each spore was borne. They 
are usually unicellular, but sometimes old conidiophores become 
se] )tate. 

The perithecia are imbedded in the tissue of the leaf, usually protruding 
sufiieiently to form a small dome-shaped pimple (Fig. 183) which is some- 
times large enough to be easily visible to the naked eye but at other times 
only discernible by the aid of a lens. They are spherical or subspherical, 
90 to 160 /j. in diameter, with a somewhat beak-like projection at the ostiole. 
Six or more simple tapering bristles 25 to 75 m in length sometimes sur- 
round the ostiole. The perithecial wall is dark olive-green to brown in 
color, with polygonal reticulations. The asci are oblong to clavate, often 
somewhat curved, 55 to 75 ju by 6 to 12 /i, without paraphyses. The 
ascospores are olive-brown, two-celled, with the upper cell somewhat 
broader than the lower, 11 to 15 p by 5 ix. There are eight ascospores in 
each ascus. 

Nomenclature 

As early as 18 19 Fries (18 19) applied the name Spiloccea Pomi to the 
conidial stage of the apple-scab fungus. Wailroth (1833) described the 
same fungus under the name Cladosporium dendriticum. Fuckel (1869) 
transferred the fungus to the genus Fusicladium and called it Fusicladium 
dendriticum (Wallr.) Fckl. Cooke (1866) described the ascigerous stages 
of the forms occurring on both apple and pear as Sphcerella incBqualis. 
Clinton (1901) notes that Winter seems to have been the first to place 
the species under the genus Venturia, since the specimens on apple leaves 
distributed in 1880 in von Thumcn's Mycotheca Universalis No. 1544 are 

(18 19) Fries, Elias. Spilocaea Pomi Fr. Nov. fl. Suec. 5: 79. 

(1833) Wailroth, F. G. Cladosporium dendriticum W. Fl. crypt. Germ. 2:4:169. 

(1866) Cooke, M. C. Sphserella inaequalis Cke. Journ. bot. 4:248-249. 

(1869) Fuckel, L. Fusicladium dendriticum (Wllr.). Symb. myc, p. 357. 

(1880) Winter, Georg. Venturia insqualis Wint. Myc. uni., vonThumen.no. 1544. 

(1901) Clinton, G. P. Apple scab. Illinois Agr. Exp. Sta. Bui. 67:123. 



Plate VIII. — Perithecial static of Venturia ineegualis 

PlG. t. — Photomicrograph of the under surface of a leaf thickly dotted with perithelia 

of Venturia inaaualis 
Fig. 2. — Same as Fig. i, except that dehiscence has occurred iii a circumscissile manner 
leaving cavities in the leaf. The saucer-shaped base of the perithelium was found in 
some of these 
Fig. 3. — -Same leaf as is shown in Fig. 1, showing perithecia on upper surface of the leaf 
PlG. }.— Photomicrograph of a perithecium in action. The perithelium teas pricked 
out of the leaf with a needle and was placed in a drop of water on a glass slide. The 
photograph was made while spores were being discharged 
PlG. 5. — Photomicrograph of a free-hand section through a perithecium. This shows 

the elongation of asci that occurs when moisture is present 
FlG. 6.— Photomicrograph of the very young scab spot shown in the upper left apple in 
Plate V, Fig. 1. Shows how the cuticle is lifted by the groioth of the fungus 




Plate VIII. - Perithecial stage of Venturia inaqualis 



V\ ait IX. —Germinating ascospores of Venturia inmqualis. Ascospores discharged from 
perithecia in old lea\ aught on agar plates and alumni to germinate 

Pig. i. ■ Average germination a', hours 

I'll.. 2. Average germination after thirty-one hours 
Pig. 3. Average germination after forty-two hours 




Plate IX. - Cerminating ascospores of V 



Plate X. — Same as Plate IX 

Fig. i. — Average germination after fifty-one hours 

Fig. 2. — Average germination after seventy-two hours 

Fig. 3. — Average germination after one hundred^ and twenty hours 

Fig. 4. — Mycelium and spores more highly magnified 










^ 



A 



















' 




4 



Plate X. — ■ Germinating ascospores of Venturia incegualis 



Scab Disease of Apples 557 

called Vcnturia ifuequalis Wiht. in litt. Aderhold (1897) also places 
this fungus in the genus Venturia, calling it V. inceqiialis (Cke.) Ad., 
apparently not knowing that it had been listed previously by Winter. 

From the work of Aderhold (1894) it is known that Fusicladium dendri- 
ticum is merely a conidial stage of a perithecia-forming fungus of the 
genus Venturia. According to present rules of nomenclature, then, the 
fungus should be known as Venturia incequalis (Cke.) Wint. 

Relationships and host plants 

Aderhold (1900) states that Fusicladium dendriticum occurs on many 
Pyrus species of the Malus group (P. spectabilis Ait., P. Kaido Sieb., P. 
floribunda Sieb., P. baccata L., P. prunijolia Willd., P. rivularis Hook., 
P. divica Mnch.). A variety, Aderhold states, is found to occur on species 
of Sorbus and apparently also on Crataegus. In regard to the two last 
named, however, Aderhold was convinced by later experiments that these 
forms are entirely distinct from F. dendriticum. 

Previous to later work of Aderhold (1903) the Fusicladiaon Crataegus 
and Sorbus were supposed to be varieties of the apple-scab fungus, and 
^thus capable of infecting the apple. In this later article the author records 
cultural experiments in which the Venturia found on Crataegus was shown 
to produce the typical conidial stage of Fusicladium crataegi Adh., a 
species distinctly different from F. dendriticum. Aderhold concludes that 
the fungus on Crataegus is entirely distinct from that on the apple, and 
that there need be no fear of Crataegus as a means of infecting the apple. 
In the same article he records experiments designed to determine whether 
Fusicladium orbiculatum Denn. or Sorbus torminalis, which is closely 
related to F. dendriticum morphologically, can infect the apple. Inocula- 
tions were made with cultures derived from the form found on Sorbus, 
with the result that abundant infection followed on 5. torminalis, but none 
on apple trees, on 5. domestica, or on Pyrus chamaemespilus. Aderhold 
states, however, that negative proof is not entirely decisive and that the 
experiments should be repeated in order to obtain a positive decision. 
The writer has not attempted cross-inoculation. 

Life history 
Perithecial stage 

It is well known to pathologists that Venturia incequalis has two distinct 
stages in its life cycle. The writer's observations on the life history of 

(1894) Aderhold, Rudolf. Die perithecienform von Fusicladium dendriticum Wal. (Venturia chloros- 
pora f. Mali). Deut. Bot. Gesell. Ber. 12:338-34-'. 

(1897) Aderhold, Rudolf. Revision der species Venturia chlorospora, inaequalis, und ditricha autorum. 
Hedw. 36: 81. 

(1900) Aderhold, Rudolf. Die Fusicladien unserer obstbaume. Centbl. bakt. 2:6:593-595. 

(1903) Aderhold, Rudolf. Kann das Fusicladium von Crataegus und von Sorbus-arten auf den apfel- 
baum ubergehn? Kaiserliches Gesundheitsamt, Biol. Abt. Land- u. Forstw. Arb. 3:436-439. 



558 Bulletin 335 

the fungus began with a study of the perithecial stage during the spring 
of 1008. L. F. Strickland, who was at that time a special student in the 
Department of Plant Pathology at Cornell University, located a crab- 
apple Uvc on the campus which furnished abundant material for a study 
of the ascigerous stage. After having become interested and having 
learned what to look for, the writer had no difficulty in finding perithecia 
in old leaves under apple trees in other localities. Many investigators 
appear to have had considerable difficulty in finding this stage of the 
fungus; but the writer is convinced, from the experience of three seasons, 
that it can be found easily almost any spring following a year of foliage 
infection if looked for carefully at the right time. Material in abundance 
was found easily in 1909, kjio, and 191 1. 

The perithecia appear most abundantly on the exposed surface of the 
leaf as it lies on the ground. They are often not easily discernible with 
the naked eye, but appear under a lens as small, dome-shaped pimples 
on the surface of the leaf. They arc sometimes confused with the fruiting 
bodies of ether fungi which are very similar in external appearance to 
the perithecia of Venturia and which are frequently more abundant than 
the latter. However, by one who is familiar with their appearance they 
can usually be distinguished from other forms on examination with a 
hand lens (Plate VIII, Figs, r, 2, and 3); the pimples commonly have 
more of a dome-like form and arc plumper than those of other fungi that 
are Likely to be confused with them. 

Development of perithecia 

The perfect stage of the fungus begins to develop in fall or early winter. 
After the scab-infected leaves have fallen and decay has set in, the 
mycelium, which during the summer docs not penetrate deeper than the 
epidermis, permeates the entire leaf tissue and sometime during the fall 
or winter begins to form perithecia. This winter development has not 
been studied very carefully, but a few notes were made at various intervals 
during the winter and spring of 1908-1909. Scabby leaves were brought 
in during the latter part of November, 1908, and parts of these leaves 
were cooked in potassium hydroxid. This did not make it possible to 
separate the epidermis from the underlying tissue, as had previously been 
the ease. It seemed as if the mycelium had already permeated the tissue 
and, as it were, sewed the epidermis fast. 

On February 26, 1909, leaves were examined and found to contain 
immature perithecia* At this datethe asci were filled with a homogeneous 
mass of protoplasm which had not yet become differentiated to form 
spores. When the perithecia were pricked out in water these imma- 
ture asci would push out through any wounds in the perithecium, but 



Scab Disease of Apples 559 

they were not seen to push out through the ostiole which was probably 
not yet open. In some cases there was so much expansion of the asci, 
due to the absorption of water which was evidently admitted through 
the wounds, that dehiscence occurred, bursting off the upper half of the 
perithecium in a manner similar to that to be described as occurring in 
the ease of mature perithecia. It would seem, then, that even at this 
early date the immature asei have the ability to absorb water and exert 
a pressure similar to that exerted later by means of which the discharge 
of spores is brought about. It is evident that in nature this premature 
action, is in some way prevented. 

Some of the leaves brought in on February 26 were kept in a moist 
chamber in the laboratory until March 20, when they were examined. 
The spores had been formed and some were evidently ripening at that 
date, since they were somewhat brown. Some shooting of spores also 
occurred. This, it will be noted, is almost one and one half month earlier 
than the same stage of development was reached under natural conditions; 
in leaves from out of doors the asci were much less mature at this date. 
In some asci the protoplasm had just begun to differentiate to form 
spores and the few spores fonned were still hyaline; in other asci the 
protoplasm still existed as a homogeneous mass in the ascus. The 
perithecia were small and inconspicuous and were likely to be overlooked. 
It was especially noticeable during 19 10 that the perithecia remained 
inconspicuous until near the end of their maturity, when they enlarged 
somewhat. It was difficult to find infested leaves until the apple blossoms 
were about ready to open. 

Clinton (1901) found signs of perithecial formation in sections of fallen 
leaves made as early as October, and could occasionally connect these 
perithecia by mycelial thread with the subcuticular mycelium. He notes 
that the perithecia usually occur on the lower side of the leaf, especially 
along the veins, and believes that they usually originate from a late 
infection on the lower side of the leaf. In the writer's observations 
during the past three seasons, perithecia have been found to occur 
abundantly on both sides of the leaf. They seem to develop mostly on 
the side that faces upward as the leaf lies on the ground. There appears 
to be a negatively geotropic tendency. 

Time of maturity of as cos pores 

In nature the ascospores usually begin to mature at or about the time 
when the apple blossoms are ready to open. In 1908 mature spores were 
found on May 4 and in 19 10 on May 1 ; the blossoms were about to open 
in each case. The exact date for 1909 is not available but it did not differ 

(1901) Clinton, G. P. Apple scab. Illinois Agr. Exp. Sta. Bui. 67:121. 



560 Bulletin 335 

materially from those given for 1908 and 19 10. The ascospores do not 
all mature at one time and the ripening process may continue for about 
one month. These points are discussed more in detail and the results 
of observations are tabulated elsewhere in this bulletin. 

Discharge of ascospores 

Turning now to a more detailed study of the mature perithecium, some 
interesting phenomena are found. During the spring of 1908 an attempt 
was made to study the mode of discharge of the ascospores. Several 
methods of observing this phenomenon were employed. First, leaves con- 
taining perithecia were sectioned and the discharge of spores from the 
asci in water on a glass slide was observed under the microscope. This 
was rather an unnatural condition, since the sectioning usually cut the 
perithecia open thus exposing the asci to free access of the water. A more 
natural method seemed to be to soften the leaves and prick out the 1 peri- 
thecia with a needle, place them in water on a slide, and observe their 
method of discharge. A photomicrograph of a perithecium thus treated 
is shown in Plate VIII, Fig. 4. Even with this method the conditions 
are somewhat abnormal, since it is impossible to prick out the perithecia 
without inflicting certain wounds which seem to admit water prematurely 
and thus premature extrusion of the asci and dehiscence of the peri- 
thecia are sometimes caused. From a study of this nature, however, 
accompanied by a study of spore discharge in situ on the leaf, much can 
be learned." 

It was noticeable throughout the study that any wounding of the peri- 
thecium induces more rapid discharge of ascospores and even induces dis- 
charge of spores from perithecia that are immature. It would seem that 
the expansion and extrusion of the asei and the discharge of spores is 
due to the absorption of water by the asci, and that premature discharge 
of the spores is prevented only by the fact that the perithecia in some 
way prevent premature or too rapid admission of water. 

Another point of interest along this line is that when the perithecia 
are wounded and the consequent abnormally rapid extrusion of the asci 
takes place, it sometimes happens that the number of asci thus forced 
into action at one time is greater than can be accommodated by the ostiole. 
Consequently a greater pressure is exerted than the perithecium can with- 
stand, and the upper half is burst off thus exposing all the asci at once. 

This was first noticed in studying perithecia pricked out of leaves and 
placed in water on the slide. In many cases these perithecia would be 
seen to dehisce as described above. In Fig. 184 is shown a camera lueida 
drawing made at different stages of the above-described process. The 
dehiscence is always circumscissile, occurring nearer to the base than to 



Scab Disease of Apples 



56i 



the osliole of the perithecium, apparently just above the point of attach- 
ment of the asci so that the latter adhere to the basal part. 

A subsequent examination of leaves containing perithecia revealed the 
fact that a circumscissile dehiscence similar to that observed under arti- 
ficial conditions occasionally occurs also in nature. Certain leaves could 
be found in which the crowns of many of the perithecia had been burst 
off, carrying with them the adhering fragments of the epidermis of the 
leaf and leaving the saucer-shaped bases of the perithecia in situ in the 





Fig. 184. — Circumscissile dehiscence of the perithecium, showing two stages in the rupture 
of the same perithecium. Outlined with a camera lucida 

leaf. A photomicrograph of the surface of such a leaf is shown in Plate 
VIII, Fig. 2. This phenomenon was found to be fairly common in 1908 
but has since been observed only occasionally. It is possible that some 
particular condition of the season of 1908 favored this method of spore 
discharge. 

Clinton (1901) makes the following statement, which is suggestive 
when considered in connection with what is stated above: " When mature 
[perithecia] are more or less loosely imbedded in the leaf tissues and at 
the time of their disappearance infected leaves often show numerous small 

(1901) Clinton, G. P. Apple scab. Illinois Agr. Exp. Sta. Bui. 67:122. 



56a Bulletin 335 

holes whore they have been imbedded." The writer has little doubt 
that the " small holes " thus observed were similar to those shown in 
Plate VIII, Fig. 2, and were the result of dehiscence as described above 
rather than of the meehaneial removal of the entire perithecium. 

McAlpine (1904), who studied this fungus in Australia, notes that "the 
perithecia or spore eases soon fall away from the dead leaves, so that by 
the middle of Oetobcr scarcely a single one could be obtained from leaves 
si ill on the ground, though the minute holes showing where they had 
been were plainly visible." Here again it seems evident that these "minute 
holes" in the leaves were the result of such dehiscence. The perithecia 
are certainly not so loosely imbedded in the leaves that they fall out 
merely from their own weight. 

Another method of studying ascospore discharge is to place glue-coated 
slides in an inverted position over moistened leaves containing perithecia. 
By this means a study was made in order to determine how high spores 
may be shot; how many may be discharged in a given time from a given 
area oi leaf surface; how soon after wetting the leaves the discharge may 
begin; how long it may continue and how long a single perithecium may 
continue to discharge spores; and whether the continued discharge, cover- 
ing a period oi several weeks during the spring and early summer, is due 
to the ripening of new perithecia or of new asci in the same perithecium. 

No spores were caught on slides placed higher than 1.1 centimeter 
above the leaf, and but few at this height. At .5 centimeter a large 
number were caught. 

From a fragment of leaf 1 centimeter square 5650 spores were discharged 
in forty-five minutes. From this it is estimated that if the surface of 
the ground beneath trees set 40 by 40 feet apart were covered with old 
leaves well infested with perithecia. there might be 8,107,200,000 asco- 
spores discharged for each tree in forty-five minutes in wet weather. Cer- 
tainly this would be sufficient to account for abundant early infection, 
even though only a very small percentage of these ascospores would 
reach the trees and actually produce infection. 

The discharge of spores begins very soon after the leaves are wet. Dry 
leaves were brought in and moistened and glue-coated slides were inverted 
above them. After five minutes the slides were removed and examined. 
A number of spores were found, showing thai -hooting begins almost 

as soon as rain begins, or within rive minutes after the leaves are wet. 

In order to determine how long a single leaf or part of a leaf might con- 
tinue to discharge spores if kept constantly wet. pieces of perithecia-bear- 
ing leaves were placed in a moist chamber under glue-coated slides and 
J examinations were made, new slides being supplied each time. 

(1904I McAlpine. D. Black spot of the apple. r. Dept. Bui. 17:10. 



Scab Disease of Apples 563 

Four of these experiments were set up on May 21, [908. On the next 
day spores were abundant on three slides and there were a few on the 

fourth. On the sound day spores were abundant on one slide and 
there were a few on the others. On the third day they were abundant 
on one slide and there were a few on one other. The fourth day showed 
similar results. On the fifth day many spores could be found on the 
slide above one piece of leaf. On the tenth day a few spores had been 
discharged from the same specimen. On the eleventh day no spores 
were found. 

These leaves were dried for eleven days and then rewet, in order to 
determine whether this might induce a reawakening of activities. The 
results were negative. No more spores were discharged from any of these 
leaves. 

It is elear from this experiment, however, that under continuously wet 
conditions an uninterrupted discharge of spores can be expected for some 
time. Under these artificial conditions a single leaf continued to dis- 
charge ascospores abundantly for ten days. It is probable that in nature 
some leaves may be found that would continue even longer than this; 
and the fact that from some leaves operations w r ould begin earlier than 
from others would lengthen the period still more. It is evident, then, 
that a rainy period sufficiently prolonged to exhaust the leaves of asco- 
spores either temporarily or permanently would rarely, if ever, occur. 
It would seem probable, however, that a frequent succession of rainy 
periods would exhaust the supply earlier in the season by hastening the 
maturity and the discharge of ascospores. Judging from observations 
made during the spring of 1908, ascospore discharge continues for about 
one month or slightly longer. It was first noted on May 4, while leaves 
gathered on June 6 were found to contain only empty perithecia. 

Further experiments were tried in order to determine whether the long- 
continued production of ascospores is due mainly to the continued ripen- 
ing of immature asci in the same perithecium or to the ripening of immature 
perithecia. Pieces of leaves each containing a single perithecium were 
cut out with the aid of a razor and a dissecting microscope. These pieces 
were kept moist under a glue-coated slide, as described above. Thirteen 
perithecia were so treated, and in no case could it be found that spores 
were discharged continuously from a single perithecium for a longer 
period than one day. In one experiment including six perithecia, three 
were active on the first day and three others on the second day, the latter 
three were evidently not quite ripe on the first day. In another experi- 
ment including seven perithecia, two were active and for the first day only. 

These experiments are not exhaustive. They seem to indicate, however, 
that individual perithecia mature at different times and thus extend the 



5'm Bulletin 335 

period of ascospore discharge over one month or more in the spring; 
but thai when discharge from a given perithecium begins, if kept con- 
stantly wet its entire contents are discharged within twenty-four hours. 

Cultural characters of the fungus 

Aderhold (1896) grew the fungus successfully on a number of artificial 
media; among these he mentions leaf and stem infusions of various plants, 
such as apple, pear, cherry, birch, syringa, and pink. He grew it also on 
fresh cucumber sap and on gelatin. The cultural characters were alike 
whether the fungus was grown from ascospores or from conidia. 

Appressoria were formed, which were at first club-shaped but the clubbed 
ends cA which later enlarged irregularly, somewhat in the form of hands 
with the lingers reaching down into the substratum. The appressoria 
became somewhat brown in color, and from them developed colorless 
hyphae corresponding to the infection tubes that occur when germination 
takes place on the host plant. Voges ( [910), in describing the formation of 
appressoria, notes the presence of a gelatinous envelope which he believes 
to be important as a means of anchorage. 

One point of special interest noted by Aderhold is that cultures of the 
fungus in spring or early summer form but little mycelium and many 
spores; while cultures made on the same gelatin in the fall produce but 
very few spores and abundant mycelial growth. This he found to hold 
true whether cultures originated from conidia or from cultures obtained 
directly from the conidia from leaves in the fall. In the hundreds of 
cultures made during the two years, only one exception to this ride was 
observed. The phenomenon is attributed to a difference in the age of the 
generation. In fall the mycelium penetrates deeply into the tissues of 
the dead leaf, forming an abundant mycelial development which in spring 
results in the formation of peritheeia. Aderhold is also of the opinion 
that the first generation of the fungus- which develops from ascospores 
forms conidia abundantly and quickly at the expense of mycelial develop- 
ment. Then, on the approach of fall, the conidial formation is retarded 
and mycelial development is increased for the purpose of favoring the 
formation of peritheeia. 

Aderhold was not able to develop the perfect stage to maturity in arti- 
ficial cultures. He states, however, that there appeared very abundantly 
in the cultures, from exhausted conidial formations, bodies which he says 
are doubtless to lie regarded as young peritheeia. 

The writer experienced no difficulty in obtaining pure cultures of the 
fungus by dilution plates from conidia and by inverting plates of agar 
about one half centimeter above moistened leaves containing peritheeia. 

iiNuti! Aderhold, Rudolf. Die Fusieladien unserer obstb&ume. Landw. jahrb. 25:888. 

"i Voges, Ernst. Die bek&mpfung des Pusicladiura. Zeitsch. pilanzenkr. 20:385-303. 



Scab Disease of Apples 565 

In the latter case the ascospores when discharged were caught by the agar. 
Germinated spores were later transferred to tubes. It was noted that 
these ascospores did not germinate so quickly as did others placed in 
water, but they grew more vigorously later. 

A rtificial inoculations 

Aderhold (1S96), in carrying out infection experiments, tried several 
methods of marking the point of inoculation so as to avoid confusion with 
natural infection. He used india ink and seveial coloring matters for 
this purpose, but found nothing else so successful as surrounding the point 
of inoculation with Von Stahl's cocoa-butter-wax mixture. When the 
leaf was all dried before applying it and the ring was not made too narrow, 
this mixture adhered well, and, if not applied too hot, did not injure the 
leaf nor prevent germination of the spores. 

The inoculations were made on the young leaves and on the fruit. From 
ascospores on leaves Aderhold obtained about thirty-three per cent infec- 
tion and from conidia about twenty-three per cent. From conidia on 
fruit he had less success ; only about eight per cent of the inoculations were 
successful. According to Aderhold 's tables conidia taken from fruit or 
from artificial cultures were less successfully used than those taken from 
scabby leaves. 

The method of infection was studied also. Aderhold observed that the 
germ tube usually enters directly over the junction of two epidermal cells 
and often where several meet, in a comer so to speak. It broadens slightly 
at the point of entrance and bores directly through the cuticle. It was 
not observed to enter through wounds. Aderhold observed not only that 
the germ tube can bore directly through the cuticle, but also that the 
conidiophores sometimes bore their way out from beneath the cuticle. 
In other cases the cuticle may be ruptured by pressure from the mycelial 
growth beneath. 

Clinton (1901) considered outdoor work unreliable because of the abun- 
dant natural infection. Accordingly he used one-year-old or two-years-old 
seedlings planted in crocks indoois and grown at a temperature higher 
than that outside. These seedlings were inoculated and kept in moist 
chambers. Inoculations made in this way were not very successful. 
Branches were cut off and artificially inoculated indoors. The leaves 
dropped in two weeks and therefore no results were obtained from this 
method . 

The writer's inoculation experiments were performed mainly during the 
spring of 1908. For most of this work ascospores obtained fiom dead 

(1896) Aderhold, Rudolf. Die Pusicladien unserer obstbaume. Landw. jahrb. 25:893. 
(1901) Clinton, G. P. Apple scab. Illinois Agr. Exp. Sta. Bui. 67:120. 



566 Bulletin 335 

leaves were used. Tt was desired to determine how readily ascospores may 
cause infection, what the period of incubation is, and how infection takes 
place. 

Leaves containing an abundance of perithecia were chopped fine in 
water and this decoction was applied with a brush. The twigs were then 
inclosed in a moist chamber. This was made of a large test tube or a lamp 
chimney, having all openings closed with cotton and some moist cotton 
left inside. 

On leaves inoculated on May 16, scab first appeared on May 24. The 
same leaves had been examined on May 23, when no infection was visible. 
It is evident that the period of incubation in this case was exactly eight 
days. By May 2S these leaves showed that many infections had taken 
place on each leaf (Plate II, Fig. 1), while only a few spots could be found 
on any uninoculated leaf. On May 3 1 ninety-eight distinct infections were 
counted on nine leaves that had been artificially inoculated, while only 
twenty-four infections could be found on twelve leaves from a branch that 
represented the most abundant natural infection found. There is no 
doubt, therefore, that the abundant infection was the result of inoculation. 
Further inoculations were made on May 26. By June 12 the leaves inocu- 
lated on May 26 were very badly infected. They had been examined three 
days previously and scab was not visible; its appearance, therefore, was 
rather sudden, and the period of incubation in this case must have been 
about fifteen days unless for some reason the infection did not take place 
as soon as the inoculation was made. 

The writer would not like to infer, without further data, that the period 
of incubation increases in length as the season advances, but this may be 
possible. As the leaves become older and the cuticle thicker, a longer 
time may be required for the fungus to work its way out through the 
cuticle. This, however, is as yet only a. suggestion. In each of the above 
experiments blossoms or young fruits were also inoculated, but they 
dropped before the time for the appearance of scab had arrived. 

Artificial infection with conidia was also attempted. Both leaves and 
young fruit were inoculated with conidia from scabby leaves at three 
different dates — June 10, June 12, and June 15. The results were not 
nearly so striking as in the earlier experiments, when ascospores were used. 
A somewhat larger number of leaf infections followed than on the uninocu- 
lated leaves, but not enough more to permit safely the drawing of con- 
clusions. Infection of fruit was not successful at this time. 

In iqio Rhode Island Greening and Baldwin apples were inoculated 
when almost full-grown. Conidia from scabby leaves were used and each 
apple was covered with moist cotton as soon as inoculated. Early in 
September a large proportion, oi the inoculated Rhode Island Greenings 



Scab Disease of Apples 567 

showed late infection. On one apple nine spots were counted, several 
others had three to five spots, and about four of the twelve inoculated 
were clean. It is true that some late infection had occurred on uninocu- 
lated fruit, but not nearly so generally as on the inoculated fruit. On 
the Baldwins some spots appeared, but the success of the inoculations 
was not so great as on the Rhode Island Greenings. These experiments 
were duplicated later with negative results. 

Method of injection 

In connection with the writer's infection experiments an attempt was 
made to determine how the germ tube pierces the cuticle of the host. 
Inoculated leaves were gathered at various dates. Some were put up in 
fixer and embedded in paraffin. Others were cooked for a short time in 
caustic potash, after which the epidermis 
could be peeled off and mounted on a slide. 
This being done, it was easy to locate some of 
the germinating spores and to trace the course 
of their germ tubes through the cuticle and 
for some distance between the cuticle and 
the epidermis. In Fig. 185 is shown a camera 
lucida drawing of an early stage of infection Fiq m ;_ Ascospore infectio „ t 
by an ascospore. In general it seems that showing the germ tube from an 

the germ tube bores directly through the aseosfon entering between the 

& - & cuticle and the e pi dermis. 

cuticle and continues to grow between the Camera lucida drawing 

cuticle and the epidermis. 

As noted above, Aderhold observed a slight broadening of the germ tube 
at the point of entrance, and the formation of appressoria. This did not 
occur in any of the cases observed by the writer. Fischer (1909) states 
that the fungus cannot enter an entirely sound fruit without a break in the 
epidermis. He believes that changes of temperature, by expanding or 
contracting the tissues of the fruit, may separate the cuticle from the 
epidermis, permitting the growth of the fungus in. such places. The 
writer cannot see that there is much ground for this opinion. It seems 
that a germ tube from a spore is able to attack a perfectly sound fruit, 
although one cannot be sure that certain injuries which are not detectable 
might not be present. 

Aderhold (1900) records experiments which lead him to believe that in 
the case of Venturia pirina the pectic compounds between the cuticle 
and the epidermal cells exert a chemotropic influence on the germ tube 
of the fungus and probably also supply nourishment. 

(1900) Aderhold. Rudolf. Die Fusicladien unserer obstbaume. Landw. jahrb. 29:562-565. 
(1909) Fischer, F. Uber die bekampfung des Fusicladium. Zeitsch. pflanzenkr. 19:432-434. 




S68 



Bulletin 335 



Time of injection 

From data recorded in Table 2 it seems evident that the first appearance 
of scab on Leaves in [908 and in 19 10, undoubtedly due to ascospore infec- 
tion ;is will be shown later, was directly traceable to certain rains which 
furnished conditions for infection; and that the dates of these rains are 
the dates of infection, thus giving approximately the period of incubation 
in each case. 

Artificial inoculations recorded above show that the period of incubation 
may vary from eight to fifteen da\ s. In the table, therefore, all rains are 
recorded which occurred within these time limits previous to the date 
on which scab appealed; and the limits are further broadened so as to 
include all rains occurring five to eighteen days previous to the appearance 
of the disease, in order to allow for any possible greater variations that 
may have occurred. 

TABLE 2. Dates and Amount of Precipitation Previous to the First Appearance 
of Scab on Leaves Due to Ascospore Infection, Including All Cases of Rain- 
fall that Could Be Responsible for Each Infection. The Dates on which 
Infection Probably Occurred Are Set in Black-face Type 



190S (first appearance on May 22) 


1910 (first appearance on May 12) 


Date of rainfall 


Amount 
of pre- 
cipitation 
(inches) 


Number 

of days 

before 

infection 

appeared 


Date of rainfall 


Amount 
of pre- 
cipitation 

(inches ) 


Number 

of days 

before 

infection 

appeared 


May 16 


15 

. 12 

•30 
. 12 
.01 
•03 
•36 
■33 
40 
.08 


6 

7 
8 

9 
1 1 
12 
13 
14 
15 
16 


Mav 2 to 3 

May 1 

April 29 

April 26 

April 25 

April 24 

April 23 


■75 

•25 
• 25 

! " 

) 1-25 


9 to 10 


M.iv m 


1 1 


May 14 


13 


May 13 


16 


May 11 


17 


May 10 


18 


May 9 


19 


May 8 




May 7 




May 6 





In 1908 the first infection appeared on May 22. As the mature asco- 
spores were found on May 4 it is safe to assume that some infection occurred 
during the rains of May 7. May 8, and May 9, since these furnished the 
first favorable conditions for infection after the spores had matured. This 
gives an incubation period of thirteen to tit teen days. Conditions favor- 
able for infection also prevailed on May 13 and May 14. in which case the 
period of incubation would be eight to nine days; this period corresponds 






Scab Disease of Apples 



569 



with inoculation experiments made early in the season. Since the spores 
were mature at the date of the earlier rains (on May 7 to May 9), there 
is no reason to believe thai the infection did not occur at that time. Infec- 
tions that appeared later probably occurred during the later rains (on 
Ma\- [3 and May 14). It is evident, therefore 1 , that the infection appearing 
on May 22 occurred during one of the two rainy periods recorded above, 
probably the earlier period I May 7 to May 9). 

In 1910, at vSodus, aseospoics began to ripen about May 1. On April 29 
and May 1 some rain fell; on May 2 and May 3 the precipitation was .75 
inch, with good conditions for scab infection. The infection appeared on 
the leaves on May 12, giving a possible incubation period of nine to thirteen 
days. The table shows that there were weather conditions which would 
permit infection earlier than this date, but the following reasons exist for 
the opinion that there was no infection prior to April 29 and probably 
none before May 1 : first, the ascospores were just beginning to ripen on 
May 1 ; and, second, spray applied on April 29 preserved the foliage from 
infection. 

The abundant appearance of scab from secondary, or conidial, infection 
is shown in Table 3 , which is constructed on the same plan as is Table 2 : 



TABLE 3. Dates and Amount of Precipitation Previous to Abundant Appear- 
ance of Scab on Leaves Due to Secondary (Conidial) Infection, Including 
All Cases of Rainfall that Could Be Responsible for Each Infection 



1908 (abundant appearance at Ithaca 
on June 28) 


1910 (abundant appearance at Sodus 
on June 7) 


Date of rainfall 


Amount 
of pre- 
cipitation 
(inches) 


Number 

of days 

before 

infection 

appeared 


Date of rainfall 


Amount 
of pre- 
cipitation 
(inches) 


Number 

of days 

before 

infection 

appeared 


June 15 


.46 
T* 
.01 

•13 
.01 

•4i 
.16 

■05 


13 
14 

18 

19 

27 
28 
29 
30 


May 25 


} •« 

•15 
.19 
. 11 
.08 

} .08 


13 
14 
16 


June 14 


May 24 


June 10 


May 22 

May 20 


June 9 


18 


June 1 


May 18 


20 


May 31 


May 10 


28 


May 30 

May 29 


May 9 

May 8 


29 
30 







*irace; an amount less than .01 inch, too small to be measured. 



In 1908, at Ithaca, abundant secondary infection appeared on June 28. 
In this case there is little doubt that the infection occurred on June 15, 
when .46 inch of rain fell. This gives a period of thirteen days for incu- 



57° Bulletin 335 

bation. It is certain thai the period of incubation was not less than thir- 
teen days, since, excepting a trace on June 10, no more rain occurred until 
June 23, only live days previous to the appearance of lesions. 

In 1 9 10, at Sodus, the first production of conidia from which secondary 
infection could be expected appeared on May 12, as shown in Table 2. 
The first rainfall after this time occuired on. May 18, with showers in the 
night and in the forenoon. Since these, rains, however, consisted of 
intermittent showers, it is probable that the foliage did not remain wet 
long enough at one time to permit abundant infection to occur. It 
seems much more probable that the abundant appearance of scab on 
June 7 was the result of infections that occurred during the rain of 
May 24 ami 25, when conditions were much more favorable for such an 
infection. In the latter case rain began to fall gently and steadily at 
4 p. m. on May 24, with heavy rain during the following night, fol- 
lowed by a cloudy forenoon on May 25 with drizzling, or misting, showers, 
and a cloudy afternoon. Since this rain furnished the first really good 
infection weather after the conidia appeared — May 12 — and since 
abundant infection became evident on June 7, it may be concluded that 
the incubation period in this case was of thirteen to fourteen days duration. 

In studying these tables, it must be borne in mind that the amount of 
precipitation is not necessarily the important factor in determining whether 
or not a certain rain permits infection. A more important factor is the 
length of time that the trees remain wet so as to allow spore germination 
(Plates IX and X). In fact it is theoretically to be expected that a heavy, 
beating rain would be less favorable to infection by the fungus, since con- 
stant washing would have a tendency either to keep many of the spores 
moving from place to place — not allowing sufficient time in any one 
position for them to become established in the tissues of the host — or to 
wash many of them to the ground. The ideal condition, for infection is a 
gentle, continued rain followed by cloudy, calm weather and a saturated 
atmosphere, in which case the spores are kept wet for a long time while in 
one position. 

Ascospores have been observed to germinate within -a period of four 
hours, but it is probable that in order to produce abundant infection the 
trees must be kept wet for eight or ten hours or even longer. Any con- 
dition tending to favor the retention of moisture after rain has ceased 
tends to favor infection by the fungus. Several factors may be men- 
tioned in this connection : dense foliage prevents prompt drying-out of the 
trees after rain lias ceased; good air drainage favors rapid drying of trees, 
for which reason orchards located on hilltops are, in general, less likely to 
be seriously attacked by scab than are those in sheltered locations where 
there is poor circulation of air; showers occurring during the day, followed 



Scab Disease of Apples 



57i 



by winds or other drying conditions, are not likely to permit infection; 
showers occurring in the evening, followed by a calm night with a humid 
atmosphere, arc likely to allow abundant infection. Conditions were 
favorable for infection on each of the dates mentioned above. 

In Table 4 is shown the relation of the development of fruit buds and 
the maturity of ascospores to early scab infection. The data on infection 
are the same as those recorded in Table 2. 



TABLE 4. Correlation of the First Appearance of Scab Due to Primary 
Infection and the Date on which this Infection Probably Occurred, with 
the Development of Ascospores and of Fruit Buds 



War 


Date 

of first 

appearance 

of scab 


Condition 
of buds 
at date 
of first 

appearance 
of scab 


Probable 

date of 
infection, 
as shown 
in Table 2 


Condition 
of buds at 

probable 
date of 

infection 


Date when 

mature 

ascospores 

were first 

found 


1908 
1910 


Mav 22 

May 12 


Petals falling 

Mostly in 
bloom 


May 7 to 9 . . . 
May 2 to 3 . . . 


Almost open- 
ing 

Ready to open 


May 4 

May 1 



From Table 4 it appears that in 1908 the first infection on leaves and 
pedicels of crab-apple appeared as the petals were falling, on May 22. 
As is shown in Table 2 the period of incubation was evidently thirteen 
to fifteen days, making the dates of infection May 7, May 8, and 
May 9. This was several days before the blossoms opened. Ascospores 
were found to be mature on May 4, which would provide the necessary 
source of infection at the time when the rains referred to in Table 2 (on 
May 7 and May 9) occurred. 

In 19 10, on apple leaves at Sodus, it is evident that the early infection 
occurred just as the blossoms were ready to open (Plate XI), that is, on 
May 2 to May 3. In this case mature ascospores were first found on 
May 1. 

From the above data it appears that the leaves and buds of the apple 
are susceptible to infection as soon as they are exposed, but that infection 
does not occur until the ascospores have matured ot until the first appear- 
ance of weather conditions favorable for infection following the maturity 
of ascospores. According to observations during the past three years, the 
spores do not reach maturity until the blossoms are either opening or just 
ready to open. It seems, therefore, that there is little danger of abundant 
infection earlier than about the time when the blossom buds show pink. 



572 



Hi i i.etin 335 



Place of primary infection 

The reason for blossom-bud leaves' becoming scabby earlier than others 
is another point relating to this early infection that is worthy of eonsidera- 
tion. Many investigators have noticed that the leaves of blossom buds 
become scabbed earlier, and are often found to be infected worse, than 
those from leaf buds; others, who have not noticed this, have noted that 
flower-bud leaves are more easily burned by a spray mixture. This 
greater susceptibility to injury in many cases is the result of previous 
scab infection. These leaves are the first to open in spring; they are 
exposed to the earliest infection, while those from the leaf buds do not 
appear until later (ten days) and thus escape it. The leaves from fruit 
buds arc 1 exposed to both ascospore and secondary conidial infection, 
while those from leaf buds are, for the most part, subject only to the 
secondary attack. 

In Table 5 is shown the same relation of the development of fruit buds 
and conidia to the secondary, or conidial, infection as appears in Table 4 
to the primary, or ascospore, infection: 



TABLE 5. Correlation of the First Abundant Appearance of Scab Due to 
Secondary (Conidial) Infection and the Date on which this Infection 
Probabl\ Occurred, with the Development of the First Crop of Conidia 
and of Fruit Buds 



Year 


Date when 
secondary 
infection 

first 

appeared 
abundantly 


Condition 
of buds 

at date of 
appearance 
of secondary 

infection 


Probable 

date of 

infection, 

as shown in 

Table 3 


Condition 
of buds at 

probable 
date of 

infection 


Date when 

conidia from 

primary 

infection 
(source of 
secondary 
infection) 
appeared, as 
shown in 
Table 2 


1908 
1910 


June 28 

June 7 


Apples I to 1 
inch in di- 
ameter 

Apples A inch 
in diameter 


June 15 

May ^4 to 25 


Apples about 5 
inch in di- 
ameter 

Petals falling 


May 22 
May 12 



As intimated above, the ascospore infection is often not severe in itself. 
It is the original source, however, of this conidial infection, which is often 
much more abundant and which usually causes most of the scabby fruit 
as well as the most abundant leaf infection. 

In Table 5 it appears that abundant leaf infection at Ithaca in 1908 
appeared on June 28, when the young apples were about three fourths inch 
in diametei. In Table 3 it is shown that the infection probably occurred 



.Scab Disease OF Apples 573 

on June 15, thirteen days previous. On June 1 g the young fruit was about 
one half inch in diameter. In 1010, at Sodus, secondary infection appeared 
on June 7, when the apples were about one half inch in diameter. In 
Table 3 it is shown that the infection in this case probably occurred on 
May 24 to 25, thirteen to fourteen days previous to the date of its appear- 
ance. At this time the trees were in full bloom and some blossoms were 
falling. 

The writer has no satisfactory explanation to offer as to why the abun- 
dant secondary infection did not appear earlier in 1908, since the first crop 
of conidia, as shown in Table 2, was produced as early as May 22 * while the 
infection did not take place abundantly until June 15. Weather condi- 
tions apparently favorable for infection occurred at intervals between May 
26 and 31. It is probable, then, that some infection did occur at this 
time, but it is apparent that the abundant infection constituted the third 
generation of the season. In 19 10, as shown in Table 3, this abundant 
infection occurred much earlier, evidently during the first continuously 
rainy period following the appearance of the first crop of conidia. In 
this case, then, the infection unquestionably represented the second 
generation of the season. 

Another method of determining the date of infection and the period of 
incubation was attempted to a limited extent during the spring of 1908. 
On May 16 certain branches were inclosed tightly in paper sacks. The 
sacks were removed on May 26 and the leaves were found to be as badly 
scabbed as uncovered leaves, showing that this infection, which had 
already been visible about two days, had taken place previous to May 16, 
thus making the period of incubation more than eight days. Some of the 
sacks were not removed until later, and these prevented abundant second- 
ary infection which appeared on the unbagged branches about June 28. 

Clinton (1901) observed in 1898 that scab first appeared about May 2, 
at which time the oldest leaves had not reached full size. In 1899 the 
disease appeared first on May 5, a few days later than in the previous 
year. Clinton notes that the later leaves were the most infected in the 
latter year, while the earlier ones from flower buds were the most infected 
in 1898. The Venturia stage was later in developing in 1899. Clinton 
gives no data in either case. In 1900 scab was first found on May n and 
did not make a general appearance until the latter part of the month. 
Clinton observed further that in 1898 scab appeared more abundantly on 
the under surface of the leaf because this surface is exposed earlier than 
the upper surface. In 1899, when the infection occurred later, the leaves 
were mostly affected on the upper side. 

* Conidia were present as soon as infection appeared. 

(1901) Clinton, G. P. Apple scab. Illinois Agr. Exp. Sta. Bui. 67:114. 



574 Bulletin 335 

As an explanation of the more abundant infection of leaves from flower 
buds in [898, Clinton suggests that infection is carried by insects and also 
that these leaves are probably the most affected because they are the first 
exposed. The same phenomenon was observed by the writei in 1909 and 
1910. It was especially marked in 19 10. The writer believes that the 
hitter explanation is the more probable one. It is evident that this infec- 
tion lias occurred in some cases before the blossoms opened, as shown in 
Table 4. and therefore before insects are likely to visit the blossoms in very 
great numbers. While it is not to be doubted that insects may carry 
spores, the writer is of the opinion that scab infection results oftener from 
wind-blown or rain-washed spores. 

The development of the two forms of this fungus as outlined by Clinton 
(1001) agrees in general with the writer's observations as recorded in the 
text. The following paragraphs are quoted from Clinton's bulletin: 

May. Scab first appears on young apple leaves ami fruit and during this month 
and June obtains its greatest foothold. 

July to September. The warm, generally dry, weather is not very favorable for 
spreading the disease to the leaves, and fruit usually sutlers but little from further 
infection. 

September and October. .Scab g^jpears to develop somewhat more abundantly 
especially on the lower surfaces of the leaves, but not necessarily in vigorous fruiting 
condition. 

October and later. On the fallen dead leaves there are signs of the formation i 
perithecia of the Venturia stage. 

October to April. Perithecia slowly develop as weather conditions prove favorable 

April and May. Perithecia with mature ascospores are now found. 

June. Venturia stage disappears. 

Late infection and scab development in storage 

While the cycle indicated above may be regarded as the usual one, it is 
to be noted that seasonal variations markedly influence the development 
of the disease. In the case of abundant fall rains accompanied by fog, 
or the occurrence of excessive dew, a late infection of scab appears. The 
disease may not appear on infected fruit until the fruit is stored. This 
phenomenon has been called to the writer's attention recently by several 
investigators and growers. It is not new, however, for Goethe (18S9) 
noted that scab developed in certain cases and new infections occurred 
after the fruit had been stored. 

Brooks (190S) reports a case of late infection on Mcintosh apples. The 
fruit when picked was apparently free from scab. Two weeks later the 
apples from an unsprayed tree were found to be very scabby. Brooks 
states further that during the winter of 1907-1908 much trouble was 
experienced in the Boston cold-storage plants and many commission men 
lost heavilv because of scab. He considers it unlikely that the disease 



Zui bekampfung des apfelrostes. Gartenflora 38 : 241. 
(1901) Clinton, G. P. Apple scab. Illinois Agr. Exp. Sta. Bui. 67:121. 
(1908) Brooks, Charles. Notes on apple diseases. New Hampshin Vgr. Exp. Sta. Rcpt. 19-20 : 372. 



Scab Disease of Apples 575 

spread through the storage plant, and thinks ii resulted either from minute 
colonics that were not noticeable at picking time or from spores lodged on 
the apples.* 

The writer was informed by B. J. Case, of Sodus, New York, of a similar 
occurrence on Rhode Island Greenings. Mr. Case stated that a succession 
of very heavy dews occurred shortly before harvest time and that this 
doubtless furnished conditions for the infection. It is probable that heavy 
dews or very gentle rains would be more effective in inducing late fruit 
infection than would washing rains, which would tend to keep the spores 
in motion over the smooth surface of the apple. At the time of the early 
infection the surface of the young fruit is sufficiently rough and hairy to 
furnish lodging-places for the spores. 

Such weather conditions, with some very gentle rains, occurred during 
the fall of 1 9 10 and considerable late infection was noted in some cases. 
This had appeared to a somewhat limited extent on the Rhode Island 
Greenings in the writer's experimental plats at Sodus which had not 
received the late application of spray. The percentage of scab on these 
plats was 17, as compared with 12 per cent on plats similarly treated but 
receiving a later application of a fungicide. The inoculation experiments 
reported earlier in this bulletin also indicated the possibility of this late 
infection and the time when it may occur. 

Morse (19 ro) reports a very serious occurrence of late infection in 
Maine. During the winter of r 907-1 908 hundreds of barrels of Maine 
apples, which were free from scab when placed in storage, were later found 
to be thoroughly covered with small black specks. Morse states that the 
entire growing and harvesting season was very wet and that the vegetative 
development of the fungus continued up to harvest time. Then the moist 
apples, covered with spores, were placed in rather warm cellars, resulting 
in the infection of the fruit and the formation of small scab spots on the 
apples in storage. 

Morse and Lewis (r9n) note an instance which would seem to indicate 
that scab infection has actually occurred in storage. It was found that 
apples lying adjacent to those that were scabby when placed in storage 
became infected. Morse notes also that Professor F. C. Sears, of the 
Massachusetts Agricultural College, has informed him that the develop- 
ment of scab on stored apples is not uncommon in Nova Scotia. 

McAlpine (1904) reports late scab infection in Australian orchards. 
The disease is said to have appeared in December and January on apples 

* Brooks gives no evidence that infection may not have taken place before the apples were gathered. 
That the fruit may have been gathered during the period of incubation is perhaps in most cases the explana- 
tion, rather than that infection occurs after harvesting. 

(1904) McAlpine, D. Black spot of the apple. Victoria Agr. Dept. Bui. 17 : 6 

(1010) Morse, W. J. Notes on plant diseases in 1908. Maine Agr. Exp. Sta. Bui. 164 : 4. 

(1911) Morse, W. J., and Lewis, ('. E. Maine apple diseases. Maine Agr. Exp. Sta. Bui. 185 : 352- 
355, 390. 



576 Bulletin 335 

thai had previously been fairly clean. This occurrence McAlpine attrib- 
utes to unusually wet weather. 

How the fungus passes the winter 

Several possibilities have been suggested in answer to this question. 
One suggestion is that conidia which may lodge on the twigs or about the 
bud scales are able to retain their vitality and to germinate when favorable 
conditions arise in spring. Another suggestion is that the stroma of the 
fungus on twigs, or even on decayed leaves or fruit, may withstand 
the winter and produce in spring a new crop of conidia to start the 
infection. 

Vitality of conidia. — Aderhold (1896) reports that, although the conidia 
of Venturia incequalis germinate very readily and quickly, they soon lose 
their power of germination. Spores kept for eight weeks between glazed 
paper would not grow on gelatin. On the other hand, Aderhold notes 
that certain hyphal threads from cultures that had apparently been dor- 
mant for three months could awaken to renewed life when placed under 
favorable conditions. Further, he adds that it is not unusual to find the 
old hyphal cells rounded off, and these in suitable media produce mycelium 
by means of hyphal threads. Aerial threads may also be used in pro- 
ducing new cultures, and in the same way bits of stroma from old spots 
are agents of reproduction. 

Ewert (1910) conducted experiments in order to determine the ability 
of conidia of various fungi to withstand low temperatures. Among these, 
conidia from the pear-scab and the apple-scab fungus, taken from diseased 
Leaves and fruits, were tested. They were subjected to three periods of 
freezing, each of six hours duration at a temperature of 16 to 5 C. The 
freezing did not at all reduce the viability of the conidia of Venturia 
pirina, while only an occasional spore of V. incequalis germinated after the 
second freezing. It would seem from these experiments that the conidia 
of V. incequalis would probably be unable to survive the winter frost. 

Hibernation of conidia. — McAlpine (1902) thought that the only source 
of infection worth taking into account, so far as Victoria (Australia) is 
concerned, is front the spores produced on the leaves or the fruit in one 
season, which may become entangled in the hairs or scales of the buds and 
may germinate when favorable conditions occur in the following spring. 
At that time McAlpine had not found in Australia any trace of the 
perithecial stage of the fungus. It is not probable that he would adhere 
to this view at present. 

(1896) Aderhold, Rudolf. Die Fusicladien unserer obstb&ume. Landw. iahrb. 25 : <oj. 

1 McAlpine, D. The fungus causing black spot of the apple and pear. Victoria Agr. Dept. 
Journ. 1 : 707. 

(1910) Ewert, Dr. Die uberwinterung von sommerkonidien pathogener ascomyceten und die wider- 
Standsfahigkeit derselben gegen kalte. Zeitsch. pflanzenkr. 20 : 138 [39. 



Scab Disease of Apples 57 7 

Lawrence (1004 s ) reports having found the scab fungus on the apexes of 
a number of fruiting spurs of both Baldwin and Rhode Island Greening 
trees. These spurs were noted in the following spring and a fungus was 
found which produced spores much like the conidia of Venturia. Lawrence 
remarks: " If these were summer spores of the scab fungus they arc pro- 
duced in such small numbers, and mature at such a time, that they will be 
killed by the spraying recommended below." 

Lawrence believes that he found conidiophores in spring on old, dead 
leaves. Those kept in moist chamber in a decoction of dead leaves for 
twenty days produced conidia on the new growth identical with those pro- 
duced on small stalks from germinating winter spores. Lawrence thinks 
there is little question but that these are true forms of the scab fungus. 
In examining many thousands of infected leaves in New York State, the 
writer has never been able to observe conidia produced on old, dead leaves 
in spring. 

Persistence of stroma on twigs. — Apple scab has been observed by 
Stewart and Blodgett (1899) to occur on the twigs of Lady apples, which 
they report as being a variety very susceptible to this form of attack. 
Clinton (1901) notes having found scab on twigs but once, on a crab- 
apple tree. He regards the ascospores as the chief means of carrying 
the fungus over winter. He has since, however, reported to Professor 
H. H. Whetzel by letter that he has observed lesions frequently on twigs 
of Fall Pippin and certain other varieties of apples. Nevertheless he does 
not state that the fungus ever winters on the twigs. 

According to Voges (1907 and 19 10) it would seem that in some sections 
of Germany apple scab may grow abundantly on twigs; while in other 
sections it is at least very rare, since Aderhold, who has without doubt 
investigated the disease more carefully and thoroughly than has any other 
person, failed to find it on the twigs. Its occurrence must certainly have 
been very rare in the region where Aderhold conducted his investigations. 
However, Voges, as cited above, states that in his locality (in Hildesheim) 
scab was as common on apple twigs as on pear twigs. He names several 
varieties that have been badly affected in his garden, among which Ribston 
is mentioned. 

The stroma of the scab spot on the twig, according to Voges, remains 
more or less dormant during the winter and produces a crop of conidia in 
spring. A section through such a spot, bearing conidia as found in March, 
is shown. 



(1899) Stewart, F. C, and Blodgett, F. H. A fruit-disease survey of the Hudson valley in 1S00. New 
York (Geneva) Agr. Exp. Sta. Bui. 167 : 283. 

(1901) Clinton, G. P. Apple scab. Illinois Agr. Exp. Sta. . Bui. 67 : 118. 

(1904) Lawrence, W. H. The apple scab in western Washington. Washington Agr. Exp. Sta. 
Bui. 04 : 1-24. 

(1907) Voges, Ernst. Ueber die schorfkrankheit dor obstbaume. Dcut. landw. presse 34 : 276—277, 

(1910) Voges, Ernst. Die bekampfung des Fusicladium. Zeitsch. pflanzenkr. 20 : 385-393. 



578 l'ii LETIN 335 

Salmon i [908) also reports finding scab on the young wood and shows 
hs o\ infested twigs. En regard to the appearance of scab on 
young wood this author states: "According to the variety oi apple 
attacked, its appearance varies considerably; in some cases e. g. on 
Coxe's Orange Pippin the diseased wood becomes somewhat swollen and 
prominently blistered, in other cases e. g. on Wellington the blistered 
appearance is loss prominent, and the shoot is not swollen. In still other 
cases the scab produces isolated characteristic markings which give the 
wood a pocked ap e. Severely attacked young shoots may, as in 

the ea.se oi' those of Lord Suffield, be blistered almost continuously over 
the surface, and the bark will then subsequently peal off in Hakes." 
Salmon considers Coxe's Orange Pippin a susceptible variety. 

The same author (1909) notes that twig infection was found to occur 
only on the following six varieties: Golden Noble, Ecklinville, Coxe's 
ge Pippin, Blenheim Orange, Warner's King, and Peasgoods Nonesuch. 
It was very severe on Coxe's Orange Pippin and on Ecklinville. 

Eriksson (1911) mem ions having frequently observed the fungus on 
twigs oi the current year's growth but not on older ones. 

O nsidering all the evidence, the writer is convinced that the ascigerous 
. is the principal agent involved in carrying the fungus over winter. 
It has been shown that twig infection is not oi very common occurrence 
and that the conidia cannot withstand winter temperatures. Further- 
more, evidence that the ascospores are the chief source of primary spring 
infection is furnished by the fact, noted more in detail in another part of 
the bulletin, that orchards the leaves of which have been plowed under or 
burned early in spring or late in fall are less seriously affected than are 
orchards in which the leaves are left exposed. 

Formation of appressoria. — There might remain the possibility of infec- 
tion resulting from the germination of appressoria produced by the 
mycelium, as described by Aderhold and already referred to. Since 
Aderhold describes these enlargements as becoming more or less brown in 
color, it may be possible that they aie able to function as resting spores. 

Summary 

The observations on scab infection given above lead in general to certain 
conclusions with reference to the time and source of infection under con- 
ditions existing in New York State: 

1. The early infection is chiefly, if not entirely, from ascospores, and it 
may appear durit first period of weather favorable to infe< 

S London Bd. Agr. J in I 3 

A/. (.".<!. (Wye, Kent). Joum. 
■s 

achate und die schi 
ai 9-131. 



Scab Disease ov Apples 570 

occurring after the ascospores have matured. Usually this does not take 
place until blossoming time or immediately before. There seems to be 
little danger that infection will occur much earlier; this is probably not 
because the leaves and buds are not earlier susceptible to seab, but rather 
because the ascospores have not matured in sufficient quantities to cause 
general infection. 

2. The period oi incubation may vary from eight to fifteen days; so 
that after this length of time has elapsed subsequent to the date of the 
earliest ascospore infection a crop of conidia is produced, from which a 
second, and usually more abundant, infection may appear eight to fifteen 
days following the first period of weather favorable to infection that occurs 
after the above crop of spores has ripened. This generation may in turn 
produce another, and so on tluoughout the season. However, the various 
infections do not always oecitr only in successive jumps at intervals of 
eight to fifteen days, as the above discussion might lead one to believe. 
The crop of ascospores are not all matured and do not all discharge at 
one time. They begin to ripen at about the time indicated above and 
furnish a constant source of infection for a month or more. Thus the 
individual infections belonging to the first generation may be started 
at several different dates and consequently produce their first crops of 
conidia at different dates. It is possible also that individual infections 
occurring at the same time do not all have the same period of incubation. 
Thus there may be a more or less constant appearance of scab, with the 
more pronounced jumps at intervals as indicated above. In fact this is 
what usually occurs. 

3. The earliest infections usually occur on the lower side of the leaves. 
This is due to the fact that the lower side is more exposed at that time, 
while the leaves are unfolding. The later infections occur more abun- 
dantly on the upper surfaces, which by that time have assumed a more 
exposed position. 

VARIETAL SUSCEPTIBILITY 

Perhaps the best method of obtaining data as to varietal susceptibility 
in case of this disease is to summarize the results that have been obtained 
and the observations that have been made by various investigators for 
years past. In reviewing this work it becomes evident that too much 
stress should not be laid on a single set of observations. It is found that 
certain varieties may be resistant in one year and susceptible in another 
year under conditions which for average varieties are as favorable to the 
disease in the one case as in the other. The experiments reported by Adcr- 
hold emphasize this point. The writer, on comparing his own results 
with those previously reported by others, finds the above statement to 
hold iiood. 



5 So Bulletin 335 

Baldwin, for example, is usually listed among resistant varieties; yet 
on the cheek trees in the writer's experiments during 19 10 Baldwin trees 
yielded ninety-eight per cent of scabby fruit, much of which was badly 
infected. In 1909, which was not an epidemic year, the Baldwins were 
as badly diseased as were the Rhode Island Greenings. The Ben Davis, 
which is usually reported as resistant, was as badly scabbed during both 
seasons as was the average variety. 

It may be possible that the reputation of these varieties is due partly 
to their color. The scab is not so conspicuous on the dark-colored fruit 
as on that of the light-colored varieties. Many growers who had very 
scabby Baldwins in the season of 19 10 considered them clean, while Rhode 
Island Greenings, which were really less diseased, were considered a 
scabby lot. However, as previously stated, it is possible that Baldwin 
and Ben Davis may be more or less resistant in some seasons, since it 
has been proved that such variations do occur. 

There seems to be no satisfactory explanation for such variations, 
although several suggestions may be offered. They may be due to a 
relation of the weather conditions or of the time when weather favorable 
to infection occurs, to certain stages in the development of the fruit and 
the leaves of certain varieties. For example, a Baldwin may be just at 
its most susceptible stage at the time when weather favorable to infec- 
tion occurs in this season, and in the next season may have passed this 
stage; while a Rhode Island Greening may so develop with respect to 
the weather as to give the opposite results each year. Therefore the Bald- 
win would be susceptible in one year in a certain locality, and the Rhode 
Island Greening in the next year. In some varieties the susceptible 
period may be reached sooner or may last longer than in other varieties; 
this explanation, however, is as yet only theoretical. 

Another plausible explanation is suggested by a few statements made 
by Bailey (1892), from whom the following is quoted: 

One other important consideration must not be overlooked here, and that is the fact 
that enemies often progress or develop as rapidly as do the host plants. I imagine that 
by the time we are able to breed scab-proof varieties — from the present standpoint — 
our scab-fungus will have developed a capability to attack more uncongenial hosts. 
This is the common history of injurious insects and fungi; they take on new habits 
to accommodate themselves to new conditions. It is possible that a good market apple 
may spring up which is for the time scab-proof; but when we have learned how to pro- 
duce such kinds with tolerable certainty, the enemy will have grown cunning too, 
I fear. How many are the pears which are sent out as blight-proof, and yet in a few 
years they suffer with the rest. We are in the habit of distrusting the originator who 
makes this claim if it turns out false in after years, but I am inclined to think that some 
of these varieties really are measurably blight-proof at first. If the histories of varieties 
of fruits could be written from the natural-history side, I fancy that many of our notions 
would be upset. 

I would iK it discourage Doctor Hoskins* efforts toward scab-proof apples, but I am 
not over-confident of success. For my generation, at least, I must pin my faith to the 
squirt-gun. 

(1892) Bailey, L. H. Scab-proof apples. Garden and forest 5 : 442, 



Scab Disease of Apples 581 

The above was written in connection with a discussion of the Baldwin 
apple. At that time this variety was considered fairly resistant to scab, 
yet Professor Bailey notes that in some cases it was attacked to some 
extent. It is significant to mention here that during the season of 19 10 
the unsprayed Baldwins in Mr. Case's orchard at Sodus, New York — 
one of the most carefully-cared-for orchards in the State — gave ninety- 
eight per cent of scabby fruit. Might it not be possible that the fungus 
has become better adapted to this host than in former years, as predicted 
by Professor Bailey in 1892 ? It is not necessarily true that the scab fungus 
in general has become able to attack more uncongenial hosts; but in this 
particular Baldwin orchard a strain of the fungus that is capable of flourish- 
ing under favorable weather conditions on the Baldwin apple may have 
been bred merely by means of natural selection. It should be possible 
for experimenters to breed fungi as well as higher plants; and in a Baldwin 
orchard in which the fungus is more or less harbored for years, those 
strains that are capable of attacking that variety persist and multiply 
until finally the disease is as prevalent as in other orchards of other pre- 
viously susceptible varieties. 

There is no reason to expect that any variety on which the fungus can 
grow, no matter to how small an extent, will remain resistant indefinitely 
if it is permitted to select, cultivate, and multiply those strains of the 
fungus that are capable of attacking it. Perhaps if an absolutely immune 
variety could be found there would be hope that such a variety could 
be kept immune until some particularly virulent strain of the fungus 
lodged on it by chance. Then, if the virulent character of the latter is 
perpetuated, there is no reason why its offspring would not be multi- 
plied, in time even producing an epidemic on this previously immune 
variety. 

Voges (19 10) claims that red-skinned varieties are much more resistant 
to scab than are closely related white varieties. He offers the suggestion 
that the coloring substance in the skin may in some way be the cause 
of the resistance to infection of such varieties. Eriksson (191 1) takes 
issue with this view and cites cases in which red-skinned apples were 
as badly attacked as others. 

Aderhold (1896) made a study of varietal susceptibility and, while 
great difference was noted, none of the four hundred and fifty varieties 
of apples cultivated in Germany were found to be immune. Aderhold 
concludes also, on comparing his obervations with those of Goethe, that 
varietal susceptibility may van" in different localities. 

(1896) Aderhold. Rudolf. Die Fusicladien unserer obstbaume. Landw. jahrb. 25 : 894. 

(1910) Voges. Ernst. Die bekampfung des Fusicladium. Zeitsch. pflanzenkr. 20 : 385-393. 

(191 1) Eriksson, J. Die rote farbe der fruchtschale und die schorfkrankheit der obstsorten. Zeitsch. 
pflanzenkr. 21 : 129-131. 



582 Bulletin 335 

The same author (1002) records the results of a five-years experiment 
on susceptibility of varieties and on the relation of weather to such sus- 
ceptibility. One hundred and sixty-three varieties were kept under 
observation during the season from 1S97 to 1901, inclusive. Notes were 
taken with reference to the disease on foliage. An elaborate tabulation 
was made showing the relative susceptibility of each variety in each season. 
Only a few varieties exhibited the same disease-resisting power throughout 
the five years, and almost without exception these were such varieties 
as suffered little during the great epidemic year of 1897. Some varieties, 
Aderhold notes, rose almost by bounds, so to speak, to a condition of 
relative freedom from disease; others were attacked very severely in 
1900 and but slightly in 1901; while still others showed gradually less 
severe attacks each year. Aderhold notes further that not a few varieties 
which in 1897 — the epidemic year — were relatively immune, were very 
susceptible in 1898 and 1S99. As a result of his investigations this author 
states that only those few varieties which were resistant every year can 
properly be considered resistant; and one can plainly see by examining 
his results that resistant varieties cannot be selected from the observa- 
tions of a single season, even though that season be an epidemic one. 
This doubtless accounts for some of the conflicting reports on resistance 
of certain varieties to scab. 

An anonymous author (1900) notes that certain varieties may appear 
to be immune in one season but may be very susceptible in another season, 
under different weather conditions. This author notes also that no variety 
can be considered entirely immune under all conditions. The same general 
sentiment is voiced by Fischer (1909), who states that no variety is immune 
and that many varieties may be practically free in certain seasons and 
yet suffer at other times. 

CONTROL 

SANITARY MEASURES 

It should not be inferred from what has been said regarding infection 
by the perfect stage of the scab fungus that the disease can be controlled 
by sanitary measures alone. There will probably always be enough fallen 
leaves exposed to permit some infection, from which further spread may 
be rapid. It is doubtless true also that spores can be carried for consider- 
able distances from neighboring orchards. If, however, the dead leaves 
are turned under, it is probable that much less danger will result from 
primary infection usually occurring about blossoming time. In most 

(1900) Anonymous. Einigc krankheiten unci feind .Mine und weinreben. Deut. landw. 

presse 27 : 721. 

(.1902) Aderhold. Rudolf. Ein beitrag zur frage der empfanglichkeit der apfelsorten fur Fusicladium 
dendriticum (Wallr.) Fuckel und deren beziehungen zum wetter. Kaiserliches Gesundheitsamt, Biol. 
Abt. Land- u. Forstw. .. Arb. 2 : 560-566. 

Fischer, F. Uber die bekampfung des Fusicl < lium. Zeitsch. pflanzenkr. 19 : 43_'-434. 



Scab Disease of Apples 583 

cases the primary attack would probably be so light as to cause no serious 
results aside from furnishing convenienl sources for bundanl 

secondary infection. Theoretically, then, it should be possible to dispose 
of many sources of primary infection by plowing the orchard in the 
previous autumn, or early in the spring before the ascosporcs have 
ripened. 

A striking demonstration of this point was observed at .Medina, New- 
York, by the writer in 1909 and reported by Whetzel (1910). Several 
orchards in a certain locality were very severely attacked by the early 
infection. None of these orchards had been plowed and the leaves were 
found to contain perithecia in abundance. One orchard just across the 
road from one of those mentioned above had been plowed early in the 
spring and the old leaves were thus turned under. This orchard was 
comparatively free from scab on the leaves at the date when these observa- 
tions were made. 

Brooks (1909) also cites a striking demonstration of this point. He 
states: "The results were secured in two Mcintosh orchards, which 
have been under observation for several years. Both have been seriously 
affected with scab each year, and the percentage of loss has been approx- 
imately the same in the two. Both orchards were in sod. About the 
middle of April a fire escaped from a sugar camp and swept over the entire 
area of one of these orchards. Not a scab spot could be found in this 
orchard the following summer, while the disease was quite common in 
the other orchard." 

Other features of sanitation — such as the removal of rotted fruit and 
dead branches, and the like — although doubtless important in insect control, 
are probably not of importance so far as scab is concerned since the prin- 
cipal winter home of the fungus is in the leaves. Drainage, of course, 
should be considered a factor in case of very wet soils, since an excess 
of moisture favors development of the fungus by increasing atmospheric 
humidity; this hinders rapid drying of trees after rains and thus creates 
favorable conditions for the development of ascospores. 

Pruning is an important factor. The denser the foliage of the tree, 
the more slowly it dries out after each rain. The longer the tree remains 
wet, the better is the opportunity for spores of the fungus to germinate 
and cause infection. 

The location of the orchard with respect to air drainage is also important 
for the same reason. Trees situated on a hill, or otherwise located where 
there is free circulation of air, dry out much more quickly than do those 
growing in a pocket into which air currents commonly do not pass. 

(1909) Brooks, Charles. Some appl ■ diseases. New Hampshire A^r. Exp. Sta. Bui. 144 : 116. 

(1910) Whetzel, H. H. Report of the committee on plant diseases. New York State Fruit Growers' 
Assn. Rept. 9 : 19-20. 



5*4 Bulletin $3$ 

selection of resistant varieties 
This 1 practice offers very little promise as a means of control. In the 
discussion of varietal susceptibility, it has been shown that no variety 
is likely to remain immune, or even very resistant, for many years. 
Further, even though a resistant variety could be obtained, the grower 
could not afford to sacrifice other desirable qualities for this one. There- 
fore it would seem better to select for quality of fruit and for productive- 
ness, vigor, and hardiness of tree, and depend on some method of pro- 
tecting the tree from scab. 

SPRAYING 

In view of the above discussion it will become evident that the most 
important method to be employed in controlling this disease is by timely 
applications of a protecting substance to the host. The early infection 
may be lessened by disposing of the dead leaves or by plowing them under. 
Proper pruning and good air drainage will aid in reducing danger of infec- 
tion, and varieties may be selected that are somewhat less susceptible 
than others. But with all these precautions it is certain that clean fruit 
cannot be grown unless the trees are properly sprayed. This is now 
generally admitted and needs no argument. Much is yet to be learned, 
however, as to methods. The study of the habits of the parasite, which 
has already been dwelt upon in the text, is of importance chiefly as a 
means of learning how, when, and where it can be most effectively attacked 
without injury to the host plant. 

The problem is to find the best fungicide and to learn how and, still 
more important, when it should be applied. 

Fungicides 

Some of the properties essential for a good protective fungicide may be 
enumerated as follows: first, the substance that goes into solution from 
the dried coating of spray material must have fungicidal value; second, 
this substance must go into solution in the presence of meteoric water, 
or it may be so brought into solution by the germinating spores of the 
fungus to be controlled, in sufficient quantity to prevent germination 
of the spores; third, it must not go into solution in such quantities or so 
rapidly that the material will all dissolve and be carried off with the first 
short rain, of in such quantities as to cause injury to the host if the result- 
ing solution is caustic; fourth, the material, if applied as a solution, must 
change to a relatively insoluble form in drying on the plant, else the results 
will not be lasting as the substance will all be washed off with the first rain; 
and fifth, the material must adhere to the plant so that the solid particles 
will not be removed mechanically bv rains or otherwise. 



Scab Disease ov Apples 585 

All these points must be considered in choosing a fungicide for the 
control of apple scab, as well as of other diseases of a similar nature when 
the principle involved is one of protection. There arc many substances 
that are powerful antiseptics and have strong fungicidal properties but 
arc not effective in controlling such diseases. Many such substances 
have been tested by reliable investigators and have proved to be inefficient. 

McAlpine (1907) reports experiments in which were used certain pro- 
prietary sheep dips (Cooper's and Little's); also phenyl, phenylene, crude 
carbolic acid, and oil of tar, 1 part in 160 parts of water. None of these 
materials were at all effective in preventing scab. 

There is no doubt that many of these substances possess fungicidal 
properties. Doubtless the reason why they do not succeed is that they 
are too easily soluble in water and do not change to an insoluble form 
after being applied. Since the fungicide protects from infection by form- 
ing a protective layer over the surface of the susceptible parts of the plant, 
there seems little reason to expect that a soluble preparation such as 
carbolic acid, which does not become relatively insoluble after drying 
on the plant, can be used in controlling such diseases as apple scab. 

In the case of bordeaux, the mixture, of which copper is the essential 
fungicidal ingredient, is applied in the insoluble form and adheres to 
the plant like paint, forming a comparatively permanent protection. 
The copper is probably made effective by being very slowly brought 
into solution by. means of certain agencies, cither atmospheric con- 
ditions or the solvent action of spores, as needed. In the case of lime- 
sulfur solution, the sulfur, which is evidently the fungicidal ingre- 
dient, is applied in a soluble form. This would be almost, if not 
completely, washed off in the first few minutes of rain, leaving the 
plant unprotected even during the latter part of the same rain. During 
the drying process, however, certain chemical changes occur and the 
sulfur is deposited as a relatively insoluble precipitate, forming a protec- 
tive and comparatively permanent layer as in the case of bordeaux mixture. 

A third type of germicide, carbolic acid, may be compared with the 
two discussed above. This is applied in the soluble form. It dries on 
the plant without becoming insoluble; consequently it is dissolved and 
carried away in the first few minutes of rain and the plant is thereafter 
left unprotected. Carbolic acid may be a stronger germicide than lime- 
sulfur or bordeaux, but the lack of the one peculiar property, a proper 
balance of solubility, makes it worthless for this purpose. 

Bordeaux mixture 

Many fungicides and many combinations of fungicides have been tried 
in connection with the control of apple scab. Until very recently bordeaux 

(1907) McAlpine, D. Experiments with black spot of apple. Victoria Ayr. Dcpt. Journ. 5 : 362-363. 



586 Bulletin 335 

mixture has remained preeminently the standard, and the only spray 
used generally foi the purpose. So much was this the case that the state- 
ment that bordeaux mixture is the best fungicide has become axiomatic. 
Most attempts at improvement in fungicides have been along the line of 
\;u ious modifications of bordeaux or by the use of other copper compounds. 
Many such combinations have been tried with varying success; the number 
of experiments of this kind is too great to permit of their mention here. 
Perhaps no one man has tried a greater number of such combinations 
than has MeAlpine (1902), to some of whose very. interesting experiments 
brief reference is here made. 

In his report on control experiments in Australia during the season 
of 1 90 1 and 1902 MeAlpine records the use of many modifications of 
bordeaux mixture, with some suggestive results. Many substances were 
used, such as linseed oil, sal ammoniac, nitric acid, alum and salt, molasses, 
caustic soda, washing soda, permanganate of potash, bluestone, and rosin. 
In addition to these was a certain " Grant's mixture," with some unknown 
substance added. This gave much better control in every one of the 
three sets of experiments than did ordinary bordeaux or any other modi- 
fications tried. The addition of common salt seemed to increase somewhat 
the efficiency of bordeaux. 

Dust sprays 

Dust sprays, if efficient, can be applied with much less expense than 
other sprays, and can be used in orchards so located topographically that 
it is difficult, to transport heavy spraying apparatus. In most cases in 
which comparative tests of dry spray and liquid spray have been made, 
a dry bordeaux preparation was used, which was applied with a special 
blowing apparatus. 

The results in general have been decidedly discouraging for the dust 
spray, as will be shown by the following reports. In many cases the dust 
spray seemed to have no effect whatever in controlling the fungus, and in 
other cases but little effect. In one State, however, the dust spray seemed 
to give fairly good results in controlling scab. 

Close (1905 and 1906) compared dust sprays with liquid sprays for 
the control of apple scab and other pests. The dust spray controlled apple 
scab very well in both seasons. The author concludes that the dust 
spray promises well for Delaware. No further experiments have been 
reported by him. 

A series of experiments covering a period of three years, comparing 

(190J1 MeAlpine, D. Experiments in the treatment of black spot of the apple and pear. Victoria 
Agr. Dept. Journ. 1:620 

(1905) Close, C. P. Dust spraying in Delaware. Delaware Agr. Exp. Sta. Bui. 69 : 1-7. 

(1906) Close, C. P. Third report on dust and liquid spraving. Delaware Agr. Exp. Sta. Bui. 76 : 
1-19. 



Scab Disease of Apples 587 

dust sprays with liquid sprays, was conducted by Crandall (1906) of 
Illinois. In these experiments a dry bordeaux was used in combination 
with pans green. The test was repeated in each of the seasons 1903, 
1904, and 1905. In every case the dust spray is reported as almost worth- 
less so far as controlling apple scab is concerned. Even the foliage was 
not protected and it fell from the treated trees about as badly as from 
those untreated. 

Faurot (1908) reports experiments in Missouri, where the use of home- 
made and commercial dust bordeaux resulted in only four to five per cent 
of scab-free fruit while the use of liquid bordeaux resulted in over ninety 
per cent. 

Lawrence (1906) obtained eighty-eight to ninety-seven per cent of 
scab-free fruit with liquid bordeaux and only one to nine per cent with 
dust bordeaux. In another experiment reported by Lawrence, liquid bor- 
deaux protected eighty-four to ninety-three per cent of the fruit and 
dust bordeaux protected only five to eight per cent. 

The examples given are sufficient to indicate that under ordinary con- 
ditions little or no dependence can be placed on a bordeaux dust spray 
such as has been used up to the present time. These examples do not pre- 
clude the possibility of finding a satisfactory dry preparation, however, 
and if some form of dust spray can be devised that will be efficient the 
cost of spraying will be much reduced. 

Lime-sulfur preparations 

Lime-sulfur solution. — The use of lime-sulfur preparations as fungi- 
cides is discussed in detail by the writer in three bulletins recently pub- 
lished by the New York State College of Agriculture at Cornell University : 
Bulletin 288, Spray injury induced by lime-sulfur preparations; Bulletin 
289, Lime-sulfur as a summer spray; Bulletin 290, Studies of the fungicidal 
value of lime-sulfur preparations. It is therefore not necessary to repeat 
that discussion here, further than to say that the work on this problem 
begun by Cordley (1908) — to whom belongs the credit of introducing on 
a practical basis in America the use of lime-sulfur solution as a summer 
spray — has progressed with increasing momentum each year since its 
beginning. The result is that to-day a large number of growers through- 
out the country have profited by the experience of the several investigators 
who have been working on the problem, and are using lime-sulfur solution 
combined with lead arsenate as a summer spray for apples. 

(1906) Crandall, C. S. Spraying appLs. Relative merits of liquid and dust applications. Illinois 

Agr. Exp. Sta. Bui. 106 : 205-242. 

(1906) Lawrence, W. H. Apple scab in eastern Washington. Washington Apr. Exp. Sta. Bui. 75: 
1-14. 

(190K) ( ordley, A. I!. The lime-sulphur spray as a preventivi of apple scab. Rural New-Yorker 
67 : 202. 

(1908) Faurot, F. W. Spraying versus dusting. Missouri Fruit Sta. Bui. 10 : 1 24, 



588 Bulletin 335 

The principal advantage of lime-sulfur over bordeaux mixture lies in 
the fact that the severe russeting of fruit often resulting under certain 
weather conditions from the use of the latter is avoided. The fruit sprayed 
with lime-sulfur usually has a much smoother, more highly colored skin 
and a more waxy finish than has that sprayed with bordeaux mixture. 
The occurrence of bordeaux injury has seemed to be more common within 
recent years than formerly, and the advent of an efficient substitute which 
promises to avoid the difficult}' has been heartily welcomed by growers. 

It must not be understood, however, that the use of lime-sulfur with 
lead arsenate is advised as the only summer spray for apples. This recom- 
mendation at present is safe for New York State, since here scab is the 
all-important fungous disease for which spraying must be done. Farther 
south, however, bitter rot and apple blotch are of vital importance, and 
Quaintance and Seott (19 12) have determined that lime-sulfur solution 
cannot be relied on to control those diseases. Where they are present 
the lime-sulfur treatment for scab should be followed by later applications 
of bordeaux mixture. 

Scott lime-sulfur. — In addition to the lime-sulfur solution which con- 
sists essentially of basic calcium sulfids, prepared by boiling together in 
water lime and sulfur in proper proportions and properly diluted, there 
has arisen the self-boiled lime-sulfur devised by Scott (1909) and used 
by him so successfully for the control of peach rot and peach scab. This, 
although highly effective for those diseases, seems rather inefficient for 
the control of apple scab, probably largely because it does not adhere 
so well as is desirable. 

Waite's modification. — An interesting set of experiments is reported 
by Waite (1910) on results obtained by the use of several modifications 
of self -boiled lime-sulfur and bordeaux mixture. The most promising 
of these was prepared by adding iron sulfate to self-boiled lime-sulfur 
mixture. This seemed to increase the efficiency of the latter, which at 
the same time retained its freedom from injurious properties. 

When to spray 
First application 

The fundamental principle on which this phase of the subject is based 
has already been discussed in connection with the study of infection. To 
know when infection occurs is to know when to spray. As has already 
been pointed out, the fungicide must be applied before the infection occurs 

(1900) Scott, W. M. Lime-sulfur mixtures for the summer spraying of orchards. U. S. Agr. Dept., 
Plant Indus. Bur. Circ. .'7 : 15 17. 

(1010) Waite, M. B. Experiments on the apple with some new and little-known fungicides. U. S. 
Agr. Dept., Plant Indus. Bur. ("ire. 58 : I-IQ. 

(ioi^i Quaintance, A. L., and Scott. W. M. The more important insect and fungous enemies of the 
fruit and foliage of the apple. U. S. Agr. Dept. Farmers bul. 492 : 23-26. 




l'i \ rK XT. — The stage of development of the host reached at the 
time of first infection in iqio. Tin's is the stage at which the first 
application of spray can be made effectively 



Scab Disease of Apples 589 

in order to prevent it. The first infection usually occurs when the 1 >1< >ss< mis 
arc about to open, or as soon thereafter as favorable weather conditions 
arise. Spraying for scab must be begun before this time if the trees ; in- 
to be insured against early infection. Since the ascospores do not mature 
until about the time when the blossom buds show pink, the first applica- 
tion may be delayed until about that time. Spraying experiments in 
19 10 added evidence to this conclusion. Although the weather from the 
time when the leaf buds first opened was such as to furnish ideal conditions 
for fungous infection, the spray applied after the buds were showing con- 
siderable pink prevented the early infection. 

The primary infection is often very light. In many cases it is not 
sufficient in itself to cause much loss. This accounts for the fact that in 
many cases the application before the blossoms open has been omitted 
without loss. Several factors may enter into the conditions governing 
this point. An abundance of dead leaves lying open under the trees, 
and the development of an abundance of perithecia in these leaves, fur- 
nish the source of infection, and wet weather at the right time furnishes 
the conditions. 

Later applications 

An application after the blossoms fall is necessary in order to protect 
the trees from later attacks, and it is also advisable, under ordinary con- 
ditions, to spray again two or three weeks later. By this time the apples 
will have grown considerably and new surfaces will have been exposed. 
Sometimes a fourth application in late July or in August is necessary 
in order to prevent late infection. 

Not only should the grower watch the conditions of the fruit buds, 
but he should also watch the weather and attempt to get the spray on 
ahead of general storm periods if possible. Many growers delay the spray- 
ing until after the rain is over if rainy weather happens to be threatening 
at the time, thinking that the rain will wash off the spray. No worse 
mistake than this can be made. It is during wet weather that the spray 
is needed to protect the trees from infection, which occurs only in the 
presence of excessive moisture. The spray does not wash off so easily as 
is ordinarily supposed. If it has twenty-five minutes in which to dry 
before any washing rain occurs it will adhere well. Any spray that 
will not stand some washing after it has once dried on the tree cannot be 
considered an efficient preventive for this disease. 

Dormant spraying 

There appears to be an opinion prevalent that winter spraying is 
important in connection with the control of apple scab. Several persons 



5QO Bulletin 335 

have advocated the substitution of the dormant spray for the application 
just before the blossoms open. 

The life history of the fungus in its relation to this point has already 
been discussed. Evidence presented in that discussion shows that the 
main source of early infection is the dead leaves. Spraying the trees 
before the leaves open cannot be expected to protect from this source 
of infection, because the leaves and the young buds which are to be pro- 
tected are not yet exposed so that the spray can reach them. 

It has been suggested that the conidia can live over winter on the twigs 
or the bud scales and that the spray applied during the dormant period 
kills them. It has been shown, however, that the conidia are not likely 
to live through the winter, and further that apple scab, unlike pear scab, 
is not of common occurrence on the twigs. 

Even though it be admitted that some infection from either of the 
above sources may occur, it would not change the facts from the practical 
standpoint. It is known certainly that ascospores are responsible for at 
least most of the early spring infection, and that it is necessary to spray 
in order to protect trees from this source. It is further known that it is 
impossible to protect trees from this source except by coating the surface 
of the parts to be protected with the spray, and this cannot be done until 
those parts are exposed. 

Spraying fallen leaves 

The question is sometimes asked, will the spray falling on the dead 
leaves beneath the tree kill the ascospores or prevent them from being 
discharged? A study of the mechanism by means of which the spores 
are discharged will answer this question. The spores, being borne in a 
closed perithecium as shown in Fig. 183 (page 555), are protected from the 
fungicide until their discharge takes place. It will be seen further, from 
the same figure, that the asci containing the spores protrude beyond the 
surface of the leaf, passing the spores safely through any coating of spray 
material that may be present without even necessitating their contact 
with it. 

Summary 

Thus far mainly the theoretical side of the question as to the time of 
spraying has been presented. If the facts show that winter spraying 
can be depended on to control, or even to prevent, early infection the theory 
must be wrong. Some persons have considered that the amount of scab 
is decreased by a winter spray; but that the winter spray cannot be 
depended on to replace any of the summer applications seems clear. 
During the seasons of 1909 and 19 10 certain trees were given this applica- 



Scab Disease of Apples 591 

tion only, a strong lime-sulfur wash being used. In both these eases it 
was impossible to detect any difference in the amount of scab on sprayed 
trees and on unsprayed trees. 

It is impossible to say what might happen in some cases, but it is certain 
that in the two cases mentioned above the dormant spray did not materially 
reduce the amount of scab. The methods that are most likely to be suc- 
cessful must be adopted. A method that has failed during two consecutive 
years is certainly not to be relied on. It should be understood that good 
results may be obtained for many seasons or under certain conditions 
when the application before the opening of the blossoms is omitted. In 
many cases a single spraying after the blossoms have fallen gives excellent 
results. The point to be emphasized is that in cases in which the early 
summer spraying is important, the dormant spray cannot be substituted 
for it. 

Before leaving the discussion of the time for spraying, the writer wishes 
to emphasize the importance of making each application at the proper 
time and of being prepared to do so. Any grower having fifty to one 
hundred acres of mature apple orchard, who expects to do all his spray- 
ing with a single outfit, will find it absolutely impossible to comply with 
the above requirements, since, as can be seen, the time limit for the most 
effective application of each spraying usually does not exceed four or 
five days. The rule for every grower, therefore, should be to provide suffi- 
cient spraying equipment to thoroughly spray his entire orchard within 
four or five days at the most. This is much more important than is gen- 
erally supposed, since the spray may entirely fail to control scab if delayed 
one or two days too long, thus permitting the infection to occur before 
the application is made. 

Effect of continued spraying 

While, as has already been pointed out, spray applied to even the 
badly diseased orchards during the first year may be expected to result 
in clean fruit it is doubtless true that continued spraying year after year 
has a cumulative effect on the vigor of the trees and thus enables them 
to set a larger crop of fruit than they could if left unsprayed. The foliage, 
being protected from the attacks of the fungus, remains healthy and vigor- 
ous; consequently the trees are able to produce a larger number of strong 
fruit buds than would otherwise be possible. 

This point is already emphasized by experiments reported by Chester 
(1898) of Delaware. In experiments conducted with the same trees for 
three successive years, those unsprayed gave remarkably small yields 

(1898) Chester, F. D. Report of the Mycologist. Experiment in the treatment of apple scab 
upon the farm of S. H. Derby, Woodside, 1897. Delaware Agr. Exp. Sta. Ann. rept. 10 : 39 (,. 



592 Bulletin 335 

in the third year while the sprayed trees maintained the same quantity 
as well as quality of yield. In this connection Crandall (1906) notes that 
when the foliage of the season is lost as a result of severe scab infection, 
the trees are likely to attempt to repair the less by pushing out leaves 
from buds that should remain dormant until spring in order to form the 
fruit crop of the next year. 

There may be exceptions, however, to the above rule. If trees have 
over-borne in one season and consequently have not set fruit buds for 
the next season, it may happen that a severe early attack of scab, by thin- 
ning the fruit during the season of over-production, may enable the tree 
to set more fruit in the following season. An instance of interest in this 
connection, in the case of pears, is reported by Beach (1895). Eight Seckel 
trees that were sprayed six times in 1893 were compared with eight other 
trees of the same variety, under similar conditions except that they were 
not sprayed in 1893, as to production in 1894. Very little difference in 
yield was noted, and also very little difference in quality. The yield of 
the trees sprayed in 1893 was slightly greater, and the quality of the fruit 
was slightly better, than of those not sprayed in that year. The author 
comments on the results as follows: " It will be remembered that the 
sprayed trees in 1893 yielded at harvest nearly three times as much fruit 
as did the unsprayed trees, so that on further reflection it is not amazing 
that they did not greatly excel the latter in yield in 1894. That they were 
enabled to excel them in quantity and nearly equal them in quality of 
yield in 1894 after the heavy crop of 1893 is really strong evidence of the 
permanent beneficial effect of spraying. The permanent injurious effects 
of the scab fungus on the unsprayed trees in 1893 was no greater, if as 
great, as the permanent injurious effects of excessive yield of the sprayed 
trees even though their foliage was kept in good condition by the spray." 
Beach concludes that " even when trees .are sprayed, large annual crops 
of fruit ought not to be expected unless they are well fed and not permitted 
to overbear." 

This, then, was a case in which the permanent injury due to scab prac- 
tically balanced that due to an over-production of fruit. In the latter 
case the injury was accompanied by the remuneration of the crop of 1893, 
while in the former case it resulted in total loss. 



(1895) Beach, S. A. Spraying pear and apple orchards in 1894. New York (Geneva) Agr. Exp. 
Sta. Bui. 84 : 33~35- 

(1906) Crandall, C S. Spraying apples. Relative merits of liquid and dust applications. Illinois 
Agr. Exp. Sta. Bui. 106 : 240. 



Scab Disease of Apples 593 

BIBLIOGRAPHY 

In preparing this bibliography the writer has attempted to make it 
as nearly as possible complete up to date. Since the writer believes the 
bibliography published by Clinton in 1901 to be adequate to that date, 
a few of the earlier references have been accepted as given by him and 
have not been verified. The writer believes that all later papers of impor- 
tance have been consulted and are listed below. 

Aderhold, Rudolf 

1894 Die perithecienform von Fusicladium dendriticum Wal. (Ven- 

turia chlorospora f. Mali). Deut. Bot. Gesell. Ber. 12:338- 
342. 

Shows the relationship between F. dendriticum on living apple leaves 
and V. chlorospora f. Mali, the permanent stage on dead leaves. 

1895 Litterarische berichtigung zu dem aufsatze fiber die peri- 

thecienform von Fusicladium dendriticum Wall. Deut. 
Bot. Gesell. Ber. 13:54-55. 

Speaks of earlier investigations connecting F. dendriticum with Ven- 
turia stage. 

1896 Die Fusicladien unserer obstbaume. Landw. jahrb. 25:875- 

914. 

Detailed accounts of the scabs of apple, pear, and cherry, and their 
relation to Venturia stage on dead leaves of these hosts. 

1897 Revision der species Venturia chlorospora, inaequalis, und 

ditricha autorum. Hedw. 36:80-83. 

Describes the different species of Venturia and gives their hypho- 
mycetous stages, placing Fusicladium dendriticum under Venturia 
inaequalis (Cooke) Ad. 

1899 Arbeiten der botanischen abteilung der Versuchsstation des 

Kgl. pomologischen Instituts zu Proskau. Centbl. bakt. 2 : 
5:521-522. 

Notes Cephalothecium roseum following pear scab. Evidently first 
note of parasitism of this fungus. Summarizes experiments at Proskau 
on varietal Susceptibility. 

1900 Die Fusicladien unserer obstbaume. Landw. jahrb. 29:541- 

588. 

Same as is briefly summarized in Centbl. bakt. 2:6:593-595. Ader- 
hold notes that the perithecia are ripe before the blossoms open and that 
the first infestation appeared about the middle of May, which would 
make an application after blossoms fall too late for sure protection. 

1900 Die Fusicladien unserer obstbaume. Centbl. bakt. 2:6:593- 

595- 

Gives list of host plants and relationships. Also recommends certain 
measures for control. 

1901 Arbeiten der botanischen abteilung der Versuchsstation des 

Kgl. pomologischen Institutes zu Proskau. Centbl. bakt. 
2 :7:66i-662. 

Finds that summer spraying, if given while leaves were young, was as 
effective as both winter and summer spraying. Winter spraying alone 
apparently gave some results. 



504 Bulletin 335 

1902 Ein beitrag zur Erage der empfanglichkeil der apfelsorten fur 

Fusicladiuin dendriticum (Wallr.) Fuckel und deren bezie- 
hungen zum wetter. Kaiserliches Gesundheitsamt, Biol. 
Abt. Land- u. Forstw. Arb. 2:560-566. 

Results of five years investigation of varietal susceptibility to scab. 
Marked difference noted for some varieties in differenl seasons. 

1903 Kann das Fusicladium von Crataegus und von Sorbus-arten auf 

den apfelbaum iibergehn? Kaiserliches Gesundheitsamt, 
Biol. Abt. Land- u. Forstw. Afb. 3:436-439. 
Cultural and cross-inoculation experiments. 

Allen, W. J. 

1911 Black spot of the apple and pear. Agr. gaz. N. S. Wales 22:015. 
Popular directions for treatment. 

Alwood, W. B. 

1893 Injurious insects and diseases of plants, with remedial measures 

for the same. Virginia Agr. Exp. Sia. Bui. 24:24. 

Mentions Fusicladium dendriticum as cause of one oi the serious apple 
diseases in this .State. 

Andre, Ed. 

1888 Les Fuseieladium et nos vergers. Revue horticole 60:246- 
247. 

Short description of apple scab. 

(Anonymous) 

1895 Apple scab. U. S. Agr. Dept. Yearbook 1804:577; 1895:587; 
1898 1896:025; 1897:673. 

Treatment for prevention of scab. 

Arthur, J. C. 

1885 Apple seab and leaf blight. New York (Geneva) Agr. Exp. 
Sta. Ann. rept. 3:370. 
Short note on damage caused by this fungus. 

Atwood, G. A. 

1907 Apple scab. New York State Agr. Dept., Hort. Bur. Inspec- 
tion bul. 1:12. 
Brief note on scab. 

Bailey, L. H. 

1892 Scab-proof apples. Garden and forest 5:442. 

States that there is no marketable scab-proof variety. Baldwin 

perhaps the least susceptible. 

1895 The recent apple failures of western Xew York. Cornell Univ. 
Agr. Exp. Sta. Bul. 84:1 34. 

Attributes to apple scab the chief cause of failures, and discusses the 
fungus ami methods of combating it. 



Scab Disease of Am es 



595 



Ballou, F. H. 

1910 Apple culture iu Ohio. Ohio Agr. Exp. Sta. Bui. 217:527- 
559- 

The scab was found to be the cause of withering anil dropping of 
blossoms in unsprayed orchards. Records of spraying experiments. 

1910 The rejuvenation of orchards. Ohio Agr. Exp. Sta. Bui. 
224:117 150. 

Reports experiments favoring the use u\ lime-sulfur as a summer spray 

for apples. 

Beach, S. A. 

1897 Wood ashes and apple scab. New York (Geneva) Agr. Exp. 
Sta. Bui. 140:665-690. 

Reports experiments of applying wood ashes to the soil as not pre- 
venting seal 1. 

Beach, S. A., and Bailey, L. H. 

1900 Spraying in bloom. New York (Geneva) Agr. Exp. Sta. 
Bui. 196:399-460. 

Spraying as soon as blossoms open seems to prevent set of fruit. If 
blossoms have been opened several days spraying seems to have no bad 
effect. Laboratory studies showed that poisons or bordeaux, or even 
lime alone, prevent germination of pollen. 

Beach, S. A., and Little, E. E. 

1907 Spraying calendar. Iowa Agr. Exp. Sta. Bui. 89:4. 

General directions for spraying for scab. 

Beach, S. A., Lowe, V. H., and Stewart, F. C. 

1899 Common diseases and insects injurious to fruits. New York 
(Geneva) Agr. Exp. Sta. Bui. 170:385-387. 
Short account of life history, and methods of prevention. 

Beach, S. A., and Paddock, Wendell 

1896 Apple scab. New York (Geneva) Agr. Exp. Sta. Ann. rept. 

i4:345-;347- 

Brief discussion of apple scab and prevention by bordeaux mixture. 

Berkeley, M. J. 

J 855 Why do pears and apples crack? Gard. chron. 1855:724. 

States that this is due to Spilocaa Pi'ini, the fructigenous form of 
Helminthosporium Pyrorum, but docs not recognize specific difference 

between the forms on apple and pear. 

Bethune, C. J. S. 

1910 Apple scab (Venturia inaequalis) and commercial lime-sulphur. 

Ontario Agr. Cob and Exp. Farm. Ann. rept. 35:34-35- 

Used lime-sulfur for apple scab with good results. 

191 1 Spraying experiments. Ontario. Agr. Col. and Exp. Farm. 

Ann. rept. 36:31. 

Reports successful use of lime-sulfur and lead arsenate. Other 
arsenicals used with lime-sulfur are likely to cause burning. 



596 Bulletin 335 

Bizzozero, G. 

1885 Fusicladium dendriticum var. minor Sacc. Fl. Vcn. crittog. 
1:510. 
Follows Saccardo in giving form on apple as variety of that on pear. 

Blair, J. C. 

1899 Spraying apple trees, with special reference to scab fungus. 
Illinois Agr. Exp. Sta. Bui. 54:181-204. 
Results of spraying, directions for making fungicide, description of 

machinery, and like information. 

1907 Fruit and orchard investigations. Illinois Agr. Exp. Sta. 
Circ. 107 : 1-58. 

Note on relative merits of liquid and dust sprays for apples. Reports 
experiments of three years, 1903-1905. Dust sprays were inefficient in 
every case. 

Bonns, W. W. 

191 1 Orchard spraying problems and experiments. Maine Agr. 
Exp. Sta. Bui. 189:33-80. 

Reviews past work on sulfur compounds for the control of apple scab, 
and records results of additional experiments, with notes and observa- 
tions on spray injury. 

Bonorden, H. F. 

1851 Fusicladium virescens. Handbuch der allgemeinen mykologie, 
p. 80. 

Describes this as a new genus and species and says in part. " Komml 
in Garten auf veredelten Apfelbaumen vor." Winter gives this as a 
synonym of the pear-seal) organism, and Bonorden "s figures certainly 
more nearly resemble that species than the one on the apple. 

Brefeld, O. 

1891 Venturia ditricha f. Piri. Unter. gesammt. mykol. 10:221. 

Shows similarity of a stage produced from above fungus to the scab 
of pear and apple. 

Briosi ana Cavara 

1892 Fusicladium dendriticum (Wallr.) Fckl. Fung, par., no. 140. 

Illustrations and specimens of this fungus, together with a short 
description. 

Brooks, Charles 

1907 Diseases of the apple. New Hampshire Agr. Exp. Sta. Rept. 

17-18:267-269. 

Note containing brief description of apple scab. 

1908 Report of the Department of Botany. New Hampshire Agr. 

Exp. Sta. Rept. 19-20:330-389. 

Notes on apple scab. Late infection. Bordeaux injury. Substi- 
tution of lime-sulfur for bordeaux. 



Scab Disease of Apples 597 

1909 Some apple diseases. New Hampshire Agr. Exp. Sta. Bui. 

144: 1 1 1-116. 

Popular description and discussion of treatment. Spray injury. 
Evidence as to a c< 1 pi »re infectii >n. 

1910 Report of the Department of Botany. New Hampshire Agr. 

Exp. Sta. Bui. 151:21-22. 

Lime-sulfur likely to require more frequent application in wel seasons 

than bordeaux. Notes on late infection and importance of spraying 
before blossoms open. 

Burrill, T. J. 

1882 Notes on parasitic fungi. Agr. rev. 2:4:86-88. 

Accounl of apple seal). 

1883 An orchard scourge. Mississippi Valley Hort. Soc. Trans. 

1 1202-207. 

Account of apple scab and the damage caused by the disease in 
Illinois. 

1901 The apple scab fungus. Illinois Hort. Soc. Trans. 34:86-97. 
A popular article on present knowledge of apple scab. 

Butz, G. C. 

1898 Apples in Pennsylvania. Pennsylvania Agr. Exp. Sta. Bui. 
43 ;i 3. 16. 
Treats of apple scab and its prevention by spraying. 

Card, F. W. 

1895 Apple-scab in Nebraska. Garden and forest 8:28. 

Writer thinks the small amount of scab found in this State is due to 
dry weather. 

Chandler, W. H. 

1909 Combating diseases and insects of the orchard. Missouri State 
Bd. Hort. Ann. rept. 3 : 34 8 - 374~3 79- 
Popular description and directions for control of scab. 

Chester, F. D. 

1888 The scab of the apple and pear. Delaware Agr. Exp. Sta. 
Bui. 3:6-7. 
Short note on above, including preventive measures. 
1895 Experiments in the treatment of peach rot and of apple scab. 
Delaware Agr. Exp. Sta. Bui. 29:18-24. 
Favorable results from spraying with bordeaux mixture. 

1897 The treatment of plant diseases in 1896. Delaware Agr. Exp. 

Sta. Bui. 34: 14-19. 
Favorable results from spraying with bordeaux mixture. 

1898 Report of the Mycologist. Experiment in the treatment of 

apple scab upon the farm of S. H. Derby, Woodside, 1897. 
Delaware Agr. Exp. Sta. Ann. rept. 10:39-45. 



598 Bulletin 335 

1899 Report of the Mycologist. Continuation of the work on the 

treatment of apple scab upon the farm of S. H. Derby, 
Woodside, 1898. Delaware Agr. Exp. Sta. Ann. rept. 
11:27-30. 

Spraying experiments with bordeaux. Tables show a large gain in 
yield through tour years of spraying. Results cumulative by enabling 
tree to produce vigorous fruit. 

1900 Report of the Mycologist. Continuation of the work on the 

treatment of apple seal) upon the farm of S. H. Derby, 
Woodside, 1899. Delaware Agr. Exp. Sta. Ann. rept. 
12:36-38. 

Describes successful spraying experiments with bordeaux mixture. 

Churchill, G. W. 

1891 Apple and pear scab. New York (Geneva) Agr. Exp. Sta. 
Ann. rept. 9=337-339- 

Describes effect of this on leaves and fruit and suggests prevention. 

Clark, J. W. 

1891 Spraying for the codling moth and apple scab. Missouri Agr. 
Exp. Sta. Bui. 13:6. 
Favorable results from spraying with bordeaux mixture. 

Clinton, G. P. 

1901 Apple scab. Illinois Agr. Exp. Sta. 6^.67:109-156. 

Botanical studies of the fungus. Studied perfect stage, and demon- 
strated its connection with Fusicladium thus confirming the work of 
European investigators. 

1904 Diseases of plants cultivated in Connecticut. Connecticut 

Agr. Exp. Sta. Ann. rept. 27:301-302. 

Brief description of the disease and directions for treatment. 

Clinton, G. P., and Britton, W. E. 

1910 Tests of summer sprays on apples and peaches in 1910. Con- 
necticut Agr. Exp. Sta. Bicnn. rept. 33-34:584-618. 

Reports on experiments using bordeaux, lime-sulfur solution, sulfo- 
cide, one-for-all, self-boiled lime-sulfur. 

Close, C. P. 

1900 Plant diseases and insect pests. Utah Agr. Exp. Sta. Bui. 
65:67-68. 

Note on appearance of scab and remedy. 

1905 Dust spraying in Delaware. . Delaware Agr. Exp. Sta. Bui. 

69:1-7. 

Found hydrated lime with copper sulfate, pulverized, and paris green 
successful. Results very promising. 

1906 Third report on dust and liquid spraying. Delaware Agr. 

Exp. Sta. Bui. 76:1-19. 

Dusting bordeaux with arsenicals versus spraying. Scab entirely 
controlled by either method. Bitter rot not well controlled by dusting. 

1906 Dtisl and liquid spraying. Delaware Agr. Exp. Sta. Bui. 
72:1-23. 



Scab Disease of Apples 599 

Cobb, N. A. 

1892 Apple scab. Agr. gaz. N. S. Wales 2:216, 492. 

Reports dis< a ie on the increase and suggests use of fungicides. 

1893 Apple scab, " Tasmanian black spot." Agr. gaz. N. S. Wales 

3:276-278. 

Short botanical account of the fungus and best fungicides for pre 
vention. 

Comes, O. 

1891 Fusicladium dendriticum Fckl. Crittogamia agraria, pp. 

394 -> 1 '7- 
Shorl account of this fungus. 

Cooke, M. C. 

1866 Sphaerella insequalis Cke. Journ. hot. 4:248-249. 

Describes this as a new species and lists it on apple, pear, and the like. 

1873 Spilocaea pomi Fr. Grev. 2:64. 

Regards this as fructigenous condition of Cladosporium dendriticum. 
1877 The hyphomycetous fungi of the United States. Buffalo Soc. 
Nat. Sci. "Bui. 3:198. 

Lists Fusicladium dendriticum as occurring in the United States on 
apple leaves and fruit. 

1891 Apple scab. Grev. 20:27-29. 

Synopsis of recent spraying experiments conducted by United States 
Department of Agriculture. 

1903 Pests of orchard and fruit garden. Roy. Hort. Soc. [London]. 

Journ. 28:6-8. 

Brief popular description of disease and fungus. Advises early spring 
application of iron sulfate, followed by bordeaux after fruit has set. 

1904 Apple and pear scab. Roy. Hort. Soc. [London]. Journ. 29: 

91-92. 

Habit and life history of fungus. Methods of control. Estimate of 
losses. 

Corbett, L. C. 

1900 Fruit diseases and how to treat them. West Virginia Agr. Exp. 
Sta. Bui. 66:204-206. 

Gives characters of apple scab and best preventive treatment. 
1900 Spraying. West Virginia Agr. Exp. Sta. 6111.70:354-355. 
Results of experiments showing that it is best to spray for scab. 

Cordley, A. B. 

1904 Apple scab. Oregon Agr. Exp. Sta. Rept. 16:38-40. 

Spraying experiments with bordeaux mixture. 
1908 Lime-sulphur spray to prevent apple scab. Better fruit 3 : 3 : 26. 
Report of Cordley's first work on apple scab, using lime-sulfur 
solutions. 
1908 The lime-sulphur spray as a preventive of apple scab. Oregon 
agriculturist 17: 178. 

Account of Cordley's first use of lime-sulfur for scab. 



600 Bulletin 335 

1908 The lime-sulphur spray as a preventive of apple scab. Rural 

New-Yorker 67:202. 

Reports successful experiments with lime-sulfur solution. Account of 
writer's tirst experiments with same for this purpose. 

1909 Lime-sulphur spray preventive of apple scab. Better fruit 

3:i°:33-35- 

Records experiments demonstrating the superiority of lime-sulfur over 
bordeaux mixture for scab. 

Cordley, A. B., and Jackson, H. S. 

1911 Orchard spraying. Oregon Agr. Exp. Sta. Circ. 13:13-15. 

Directions for the use of lime-sulfur solution. 

Costantin, J. 

1888 Fusicladium. Les mucedinees simples, pp. 69-71. 

Gives figures and short account of apple scab. 

Craig, John, and Van Hook, J. M. 

1902 Pink rot, an attendant of apple scab. Cornell Univ. Agr. Exp. 
Sta. Bui. 207:157-171. 

Cephalothecium roseum gains entrance through wounds caused by the 
scab fungus. 

Crandall, C. S. 

1906 Spraying apples. Relative merits of liquid and dust applica- 
tions. Illinois Agr. Exp. Sta. Bui. 106:205-242. 

An elaborate set of experiments comparing dust and liquid sprays for 

Scan. 

1909 Bordeaux mixture. Illinois Agr. Exp. Sta. 6^.135:199-296. 

An extensive investigation of bordeaux mixture as used for apple 
scab and other plant diseases. 

Crawford, F. S. 

1886 Report on the Fusicladiums, the codlin moth, and certain 
other fungus and insect pests attacking apple and pear 
trees in South Australia. Part I. The apple and pear 
scab fungi, pp. 7-31. 

A popular discussion of the disease, estimates of losses, notes on 
varietal susceptibility, and the Like. Notes on spraying experiments. 
Suggests use of carbolic acid vapor by placing vessels of the acid in 
the orchard, as was reported by Mr. Storck to successfully control 
coffee-leaf disease. 

Cuboni, G. 

1892 Sulla forma ibernante del Fusicladium dendriticum. Soc. Bot. 
Ital. Bui. 1892:287-288. 

Describes a hibernating condition or stroma found on twigs that 
under proper conditions of moisture and heat gave rise to the char- 
acteristic conidia. 



Sewn Disease of Apples 6oi 

Cummings, M. B. 

1909 Apple orchard survey of Niagara county. Cornell Univ. Agr. 
Exp. Sta. Bui. 262:277-320. 

Curtis, M. A. 

1867 Spilocsea Pomi Ft. North Carolina Gcol. and Nat. Hist. 
Survey. Rept. 3:121. 
Lists fungus as common on skin of apples. 

Detmers, Freda 

1891 Apple scab. Ohio Agr. Exp. Sta. 6111.4:9:187-192. 

Short description of apple-scab fungus and its injury. 

Dickens, A., and Headlee, T. J. 

191 1 Spraying the apple orchard. Kansas Agr. Exp. Sta. Bui. 
174:253-292. 
Extensive spraying experiments. 
191 1 Spraying apples. Kansas Agr. Exp. Sta. Circ. 15:1-8. 

Spraying experiment comparing lime-sulfur and bordeaux mixture. 
Lime-sulfur treatment recommended where blotch is not present. 

Duggar, B. M. 

1909 Apple scab and pear scab. Fungous diseases of plants, pp. 
264-271. 

A brief account of the disease, including life history, distribution, 
economic importance, varietal susceptibility, and treatment. 

Ellis, J. B. 

1892 Dothidea pomigena Schw. The North American Pyrenomy- 

cetes, p. 605. 

States after examination of Schweinitz' specimen that this is appar- 
ently the fructigenous form of Fusidadium dendriticum. An examina- 
tion by Clinton, however, shows this not to lie the ease. 

Emerson, R. A. 

1905 Apple scab and cedar rust. Nebraska Agr. Exp. Sta. Bui. 
88:1-21. 

Results and discussion of spraying experiments. Notes on varietal 
susceptibility. 

1007 Spraying demonstrations in Nebraska apple orchards. Ne- 
braska Agr. Exp. Sta. Bui. 98:1-35 

Spraying experiments in six Nebraska apple orchards. Notes on 
cost and income. 

Emerson, R. A., Howard, R. F., and Westgate, V. V. 

191 1 Spraying as an essential part of ■ profitable apple orcharding. 
Nebraska Agr. Exp. Sta. Bui. 119:1-26. 
Commercial spraying experiments. 



602 Bulletin 335 

Eriksson, Jakob 

1885 Bidrag till kacnnedomen om vara odlade vaxters sjukdomar. 

K. Landtbr. Akad. Exptlfalt. Mcddcl. 1:61. 

1886 Der schorf dcr obstbaumc. Bot. centbl. 26:345-347. 

Abstract of an article published by Erikkson the year before, on 
scab fungus and damage caused by it in Sweden. 

1903 Om frukttradsskorp och frukttradsmogel samt mcdlcm till 
dessa jukdomars bekampandi (1. Apfelradszkorp Venturia 
dendritica) och parontradss'korp (V. pirina). K. Landtbr. 
Akad. Handl. och Tidskr. 42:53-71. 

Gives description of disease and of fungus, and suggests method of 
control. 

191 1 Die rote farbe der fruchtschale und die schorf krankhcit der 
obstsorten. Zeitsch. pflanzenkr. 21:129-131. 

Takes issue with Voges, who claims that red-skinned apples are more 
resistant to scab than are green-skinned ones. Notes having fre- 
quently observed the fungus on twigs of the current year's growth but 
not on older ones. 

Eustace, H. J. 

1902 A destructive apple rot following scab. New York (Geneva) 
Agr. Exp. Sta. Bui. 227:367-389. 

Cephalothecium roscum following scab, causing pink rot. Proved its 
parasitism by inoculation. 

1908 Investigations on some fruit diseases. New York (Geneva) 
Agr. Exp. Sta. Bui. 297:47-48. 

Scab spots enlarged under coating of bordeaux mixture. 

Evans, W. H. 

1893 Apple scab. Handb. exp. sta. work 1893:18. 

Short description of scab and means for its prevention. 

Ewert, Dr. 

191 Die tiberwinterung von sommerkonidien pathogen er ascomy- 
ceten und die widerstandsfahigkeit derselben gegen kalte. 
Zeitsch. pflanzenkr. 20:138-139. 

Subjected conidia of Fusicladium pirinum and F. dendriticum to low 
temperature (16 to 5 ) three times, six hours each time. Spores 
of the pear-scab fungus retained their normal vitality, while but very 
few of the apple-scab spores germinated after the second freezing. 

Fairchild, D. G. 

1892 Treatment of apple scab at Brockport. U. S. Agr. Dept., Veg. 
Path. Div. Bui. 3:62. 

Records negative results from spraying with different fungicides 
because of the absence of scab. 

1894 Bordeaux mixture as a fungicide. U. S. Agr. Dept., Veg. 

Path. Div. Bui. 6:43-44. 

Brief historical accounl of the use of bordeaux mixture for apple 
scab. 



Scab Disease of Apples 



603 



Farrand, T. A. 

1905 Report of South Haven sub-station for [904. Michigan Agr. 

Exp. Sta. Spec. bul. 30:474 475. 
I )us1 sprays inferior to liquid. 

Faurot, F. W. 

1903 Report on fungous diseases on cultivated fruits. Missouri 
Fruit Sta. Bul. 6:4. 
Brief note on apple scab. 

1906 Demonstration spraying for bittei rot and codling moth. 

Missouri Fruit Sta. Bul. 15: c6. 

Note on apple scab and combination treatment For scab, bitter rot, 
and codling moth. 

1908 Spraying versus dusting. Missouri Fruit Sta. Bul. 19:1-24. 

Dusting was not effective. 

Fischer, F. 

1909 ETber die bekampfung des Fusicladium. Zeitsch. pflanzenkr. 

19:432-434. 

Considers infection occurs through wounds in epidermis. Advises 
first application of bordeaux before leaves open. Notes on infection 

and varietal susceptibility. 

Frank, A. B. 

1880 Fusicladium dendriticum Fckl. Die krankheiten der pflanzen, 
pp. 587-589. 

Short description of this fungus. 

Fries, Elias 

1819 Spilocaea Pomi Fr. Nov. fl. Suec. 5:79. 

Names this fungus here for the first time rather than in Syst. Myc. 
1825 Spilocaea. Syst. orb. veg. 1:198. 

Describes this genus and gives host as living apple. 

1829 Spilocaea epiphylla. Syst. myc. 3:504. 

Describes a fungus on leaves from France that to all appearance is 
leaf form of apple (and pear?) scab. 

1829 Spilocaea Pomi. Syst. myc. 3:504. 

Describes the form of apple scab found on the fruit. 

Fuckel, L. 

1869 Fusicladium dendriticum (Wllr.). Symb. myc, p. 357. 
Renames Cladosporium dendriticum (Wallr.) as above. 

Funk, J. H. 

1910 Spraying, the sheet-anchor of success. Pennsylvania State 
Agr. Dept. Bul. 197:85-94. 
Practieal directions for spraying apple orchards. 



604 Bulletin 335 

Galloway, B. T. 

1887 Diseases of the apple caused by fungi. Missouri State Hort. 
Soc. Ann. rept. 29:297-299. 
Discusses scab and the use of the fungicides then known. 

1889 Apple scab. U. S. Agr. Dept. Ann. rept. 1889:405-412. 

Reports on spraying experiments conducted under the writer's 
direction by Goff and Tail. 

1890 Notes on the fungus of apple scab. Michigan Agr. Exp. Sta. 

Bui. 59: 28-29. 
Brief description of the fungus. 
1892 Treatment of apple, pear, peach, plum, cherry, and quince 
diseases in the orchard. U. S. Agr. Dept. Ann. rept. 
1891:362-364. 

Brief description of spraying experiments conducted under the writer's 
direction by Goff, bordeaux mixture proving more efficient than copper 
carbonate. 

1892 Experiments in the treatment of apple scab in Wisconsin. 

U. S. Agr. Dept., Veg. Path. Div. Bui. 3 13 1-36. 
Reports more fully experiments of Goff in 1891. 

Galloway, B. T., and Southworth, E. A. 

1889 Treatment of apple scab. Journ. myc. 5:210-214. 

Results of spraying experiments conducted in Wisconsin and Michigan 
with six fungicides, of which eau celeste and ammoniacal solution of 
copper carbonate were the most effective. 

Garman, H. 

1890 The apple scab fungus. Kentucky Agi. Exp. Sta. Ann. rept. 

2:46-49. 
Description of fungus and discussion of fungicides. 

1893 Bordeaux mixture for apple pests. Kentucky Agr. Exp. Sta. 

Bui. 44:25-26. 

Favorable results from spraying with bordeaux mixture for apple 
scab. 

1894 Apple-scab. Kentucky Agr. Exp. Sta. Ann. rept. 6:53-54. 

Very favorable results from spraying a Janet apple tree with bordeaux 
mixture. 
1908 Apple orchard pests in Kentucky. Kentucky Agr. Exp. Sta. 
Bui. 133:66, 70-71- 

Advises spraying before leaves expand, in bad eases. Note on bor- 
deaux and bordeaux dust. 

Goethe, R. 

1887 Weitere beobachtungen liber den apfel und birnenrost. Gar- 
tenfiora 36:293-299. 

Notes the relationship of Fusicladium stage of scab on apples and 
pears to mature stage on the dead leaves. 



Scab Disease of Apples 605 

1888 Zur bekampfung dcs aptclrostes. Gartenflora 37:263. 

Treats of the use and strength of bordeaux mixture as fungicide for 
apple scab. 

1889 Zur bekampfung des aptclrostes. Gartenflora 38:241. 

Notes that scab developed on apples in storage. Suggests the use 
of sulfur in the storeroom. 

Goff, E. S. 

1886 An experiment for the prevention of apple scab. New York 
(Geneva) Agr. Exp. Sta. Ann. rept. 4:260. 

Favorable results from spraying a crab-apple tree with hyposulfite 
of soda. 

1888 Applications for the prevention of apple scab. New York 

(Geneva) Agr. Exp. Sta. Ann. rept. 6:99-101. 

Very beneficial results from spraying parts of a crab-apple and a 
pear tree. 

1889 Experiments in the treatment of gooseberry mildew and apple 

scab. Journ. myc. 5-35~37- 
Results of spraying against apple scab with several fungicides. 

1890 Treatment of apple scab. Joum. myc. 6:19-21. 

Recommends the use of ammoniacal solution of copper carbonate. 
1890 Report on the treatment of apple scab. U. S. Agr. Dept., 
Veg. Path. Div. Bui. 11:22-28. 

Results of spraying with different fungicides, of which ammoniacal 
copper carbonate proved the most effective. 

1890 Prevention of apple scab. Wisconsin Agr. Exp. Sta. Bui. 

23:1-11. 

Description of scab, and results of spraying experiments under the 
direction of the United States Department of Agriculture, recommend- 
ing ammoniacal solution of copper carbonate. 

1891 Experiment in the treatment of apple scab. Journ. myc. 7: 

17-22. 

Reports favorably on ammoniacal solution of copper carbonate and 
ammoniacal copper sulfate. 

1892 Experiment in the treatment of apple scab. Wisconsin Agr. 

Exp. Sta. Ann. rept. 8:160-161. 
Summarizes results of spraying experiments conducted in 1890. 

1893 Preventive treatment for apple scab, etc. Wisconsin Agr. 

Exp. Sta. Bui. 34:1-13. 
Recommends bordeaux mixture. 

1893 Experimental treatment for apple scab. Wisconsin Agr. Exp. 

Sta. Ann. rept. 9:264, 270-271. 

Summarizes the results of spraying experiments conducted in Wis- 
consin. 

1894 The apple scab and its prevention. Wisconsin Agr. Exp. Sta. 

Ann. rept. 10:228-240. 

Gives note on scab and results of .spraying experiments conducted 
during several years, especially those of 1892. Recommends bordeaux 
mixture. 



606 Bulletin 335 

Gossard, H. A. 

1908 Spraying apples. Ohio Agr. Exp. Sta. Bui. 191:103-125. 

Concludes thai " orchards sprayed with lime-sulfur wash in winter 
do not need treatment with bordeaux mixture before blossoming, unless 
this ingredient is omitted from the spray applied just after bloom- 
ing." 

1909 Apple spraying in 1908. Ohio Agr. Exp. Sta. Circ. 95: 1-8. 

When lead arsenate alone was used for the first application, Wine- 
saps fell almost as soon as set, due to scab disease. 

191 1 Commercial apple orcharding in Ohio. Ohio Agr. Exp. Sta. 
Circ. 112:1-15. 

Reports successful control with lime-sulfur solution. 

Green, W. J. 

1 89 1 The spraying of orchards. (1) Spraying to prevent apple 
scab. Ohio Agr. Exp. Sta. Bui. 4:9:193-212. 
Records extensive spraying experiments. 
1898 Fruit notes. Ohio State Hort. Soc. Ann. rept. 31:11-12. 

Shows very beneficial results from spraying with bordeaux mixture 
for the control of scab. 

Green, W. J., Selby, A. D., and Gossard, H. A. 

191 1 Orchard spraying suggestions for 191 1. Ohio Agr. Exp. Sta. 
Circ. 109: 1—3. 

Brief directions for spraying. 

Halsted, B. D. 

1894 Decays of mature apples. New Jersey Agr. Exp. Sta. Ann. 

rept. 14:369-370. 

Brief note on appearance of apple scab. 

1895 Some of the more injurious fungi to fruits in 1894. New 

Jersey Agr. Exp. Sta. Ann. rept. 15:324. 
Notes scab as abundant in New Jersey. 

Hamilton, J. 

1903 Black spot. Queensland agr. journ. 13:555. 

Reports good results with the following formula: copper sulfate 4 
pounds, alum 2 pounds, lime 3 pounds, water 50 gallons. 

Harvey, F. L. 

1889 Apple scab or black spot. Maine Agr. Exp. Sta. Ann. rept. 

1888: 149-151. 
Short account of apple scab and means of prevention by spraying. 

1890 Apple scab. Maine Agr. Exp. Sta. Ann. rept. 1889:182-184. 

Notes on spraying experiments conducted by the United States 
Department of Agriculture. 

Hatch, A. L. 

1891 Experiments in treating apple scab. Journ. myc. 7:26. 

Speaks of necessity of early spraying and suggests more dilute solu- 
tions than were then used. 



Scab Disease of Apples 607 

Henderson, L. F. 

1899 Apple scab in the Potlatch. Idaho Agr. Exp. Sta. Bui. 20: 
77-95- 
Favorable results from the use of bordeaux mixture. 
1904 Some experiments with fungus diseases in 1903. Idaho Agr. 
Exp. Sta. Bui. 39:263-271. 
Spraying experiments reported. 

1906 Incomplete experiments for 1905. Idaho Agr. Exp. Sta. 

Rept. 1905:14-16. 
Spraying experiments reported. 

1907 Mixed sprays for apple scab and codling moth. Idaho Agr. 

Exp. Sta. Bui. 55:1-27. 

Ineludes studies concerning times for application of spray. 

Hoffman, H. 

1863 Cladosporium dendriticum. Index fungorum, p. 37. 
Gives references to scab under this name. 

Hoskins, T. H. 

1892 Orchard spraying. Garden and forest 5:370-371. 

Speaks of need of selection in order to obtain a more hardy variety 
of apple, resistant to scab. 

Huber, Karl 

1908 Fusicladium-bekampfung durch kupperkalkbruhe oder karbo- 

lineum. Deut. obstbaume ztg. 1908:382-387. 
Found carbolineum much inferior to bordeaux for controlling scab. 

Jackson, H. S. 

1913 Apple diseases. Oregon Agr. Exp. Sta. Bienn. crop-pest and 
hort. rept. 1911-1912:238-241. 

Jarvis, C. D. 

191 1 Apple growing in New England. Connecticut (Storrs) Agr. 
Exp. Sta. Bui. 66:256-261. 

Advises caution in the use of lime-sulfur for scab; tested on suscep- 
tible varieties such as Fall Pippin and Fameuse. Directions for treat- 
ment. 

Johnston, T. Harvey 

1910 Notes on some plant diseases. (C. A scab on apples.) Agr. 
gaz. N. S. Wales 21:563-566. 

The author refers to a scab due to Coniothecium chromatosporum. 
The accompanying illustration shows typical frost-banding. 

Jones, L. R. 

1892 The prevention of apple and pear scab by spraying. Vermont 
Agr. Exp. Sta. Ann. rept. 5:132-133. 

Reports somewhat favorable results from spraying Greening apples 
with bordeaux mixture and ammoniacal copper carbonate. 



608 Bulletin 335 

1892 Plant diseases. Vermont Agr. Exp. Sta. Bui. 28:30-34. 

Reports strong bordeaux mixture more effective than ammoniacal 
solution of copper carbonate, but injuring the foliage somewhat. 

1893 Apple scab. Vermont Agr. Exp. Sta. Ann. rept. 6:82-83. 

States that spraying experiments were failures, due to excessive rains. 
1895 Spraying orchards and potato fields. Vermont Agr. Exp. 
Sta. Bui. 44:83-93. 

Describes scab and gives favorable results from spraying with bor- 
deaux mixture. 

Jones, L. R., and Morse, W. J. 

1902 Scabbing and russeting of apples in 1Q02. Vermont Agr. 

Exp. Sta. Ann. rept. 15:230-231. 

Wet season of 1902 gave serious scab infection and induced much 
russeting of fruit and some spotting of foliage. 

1903 Occurrence of plant diseases in Vermont in 1903. Vermont 

Agr. Exp. Sta. Ann. rept. 16:154. 

Practically no loss from scab. Early season very dry, followed by 
rainy season. 

Jones, L. R., and Orton, W. A. 

1898 Spraying for the prevention of apple scab in 1897. Vermont 

Agr. Exp. Sta. Ann. rept. 11:195-198. 

Report very favorable results from spraying with bordeaux mixture, 
and show that sprayed trees hold fruit better than do unsprayed trees. 

1899 Spraying for the prevention of apple scab. Vermont Agr. 

Exp. Sta. Ann. rept. 12:156-159. 
Favorable results from the use of bordeaux mixture. 

Keffer, C. A. 

1894 Spraying apple trees. Missouri Agr. Exp. Sta. Bui. 27:1-24. 

As result of numerous experiments, bordeaux mixture reported as 
largely preventing apple scab. 

Kinney, L. F. 

1892 The use of fungicides- in the treatment of the apple scab. 
Rhode Island Agr. Exp. Sta. Bui. 15:21-22. 
Quotes from Green as to methods and results of spraying. 

1895 The scab of the apple and pear. Rhode Island Agr. Exp. Sta. 

Ann. rept. 7:185-187. 
Brief report on nature of scab. 

Kirk, T. W., and Cockayne, A. H. 

1908 Plant pathology. Apple scab (Fusicladium dendriticum) and 
apple Coniothecium (C. chromatosporum). New Zealand 
Agr. Dept. Ann. rept. 16:110-111. 

Notes on apple scab and a disease often confused with it, said by 
the authors to be caused by Coniothecium chromatosporum. The latter 
is illustrated, and is apparently identical with russeting of fruit as 
caused by bordeaux mixture or weather conditions. 



Scab Disease of Apples 609 

Kock, G. 

191 1 Schorf, Monilia, und weissfleckigkeit auf vcrschicc4cncn obst- 
sorten. Oesterr. Landw. Versuchsw. Zeitsch. 14:209-213. 
Varieties resistant and varieties susceptible to scab. 

Lamson, H. H. 

1892 Spraying against pear and apple scab. New Hampshire Agr. 
Exp. Sta. Ann. rept. 3-4:217-218, 238-239. 

Gives short description of scab and reports favorable results from 
spraying with bordeaux mixture and ammoniacal solution of copper 
carbonate. 

1894 Some fungus diseases of plants and their treatment. New 

Hampshire Agr. Exp. Sta. Bui. 19:7-11. 

Describes scab and gives favorable results from spraying with bor- 
deaux mixture. 

1895 Spraying experiments in 1894.' New Hampshire Agr. Exp. 

Sta. Bui. 27:5-7. 
Favorable results from use of bordeaux mixture. 

1903 Fungous diseases and spraying. New Hampshire Agr. Exp. 

Sta. Bui. 101 : 59-60. 

Brief description of disease and directions for spraying. Notes on 
susceptibility. 

Lawrence, W. H. 

1904 The apple scab in western Washington. Washington Agr. Exp. 

Sta. Bui. 64: 1-24. 

Description of fungus. Notes on twig infection and wintering over 
of conidia. Also production of conidia from mycelium in dead leaves. 
Distribution, varietal susceptibility. Inoculation experiments not suc- 
cessful. .Spraying experiments. 

1906 Apple scab in eastern Washington. Washington Agr. Exp. 

Sta. Bui. 75: 1-14. 

Liquid bordeaux versus dust. 

1907 Some important plant diseases of Washington. Washington 

Agr. Exp. Sta. Bui. 83:25-28. 

Popular description and methods of control. 

Link, H. F. 

1825 Spilocaea Pomi. Species plantarum, Linne, 6:2:86-87. 

Description of the fructigenous form of apple scab. 

Lloyd, J. W. 

1907 Spraying for the codling moth. Illinois Agr. Exp. Sta. Bui. 

114:377-429- 
Investigations on combined spraying for scab and codling moth. 

Lochhead, William 

1903 Some injurious insects and fungous diseases of the year 1902. 
Ontario Agr. Col. and Exp. Farm Ann. rept. 28:26. 
Brief note on prevalence of scab and directions for spraying. 



610 Bulletin 335 

1905 Insects and fungus diseases. Ontario Agr. Col. and Exp. 
Farm. Ann. rept. 30:43. 
Key to the fungous diseases of the apple. 
1909 Three important fungous diseases of the orchard. Quebec 
Society for the Protection of Plants from Insects and Fun- 
gous Diseases. Ann. rept. 1:53-55. 
Popular description of disease and fungus. 

Lodeman, E. G. 

1892 Spraying apple orchards in a wet season. Cornell Univ. Agr. 

Exp. Sta. Bui. 48:357-393. 

Gives results of spraying with bordeaux mixture, a table of varieties 
showing their susceptibility to scab, and the chemistry of bordeaux 
mixture. 

1893 The spraying of orchards. Cornell Univ. Agr. Exp. Sta. Bui. 

60: 265-286. 
Results of spraying experiments agairrst apple scab. 
1895 The spraying of orchards. Cornell Univ. Agr. Exp. Sta. Bui. 
86: 101-125. 

Gives suggestions concerning and results of spraying apples, chiefly 
against scab. 

Lounsbury, C. P. 

1905 Fusicladium of the apple and pear. Cape Good Hope agr. 
journ. 27: 169-174. 

Notes on distribution and injury in Australia and South Africa. 
Author thinks disease was introduced by nursery stock; possibly by 
diseased apples, but not so likely. Author thinks disease is carried 
over mostly on twigs. Old leaves not important. 

1908 The Fusicladium disease of the pear and apple. Cape Good 
Hope agr. journ. 33:16-32. 

Conditions favoring disease and effect on host are described. Author 
notes that apple scab has a much more limited distribution in South 
Africa than has pear scab. Notes burning of foliage by use of copper 
soda spray. 

Lovett, A. L. 

1911 Spray calendar. Oklahoma Agr. Exp. Sta. Buh 92:1-16. 

Directions for spraying. 

McAlpine, D. 

1902 Experiments in the treatment of black spot of the apple and 
pear. Victoria Agr. Dept. Journ. 1:620-630. 
Tests of bordeaux with addition of several other substances. 
1902 The fungus causing black spot of the apple and pear. Victoria 
Agr. Dept. Journ. 1:703-708. 

First observation, and history, of scab in Australia. Life history 
of fungus, symptoms of disease, and varietal susceptibility are discussed 
briefly. 



Scab Disease of Apples 6ii 

1902 Report of the Vegetable Pathologist. Victoria Agr. Dept. 

Journ. 1:803-804. 

Notes 011 spraying for control of black spot of apples and pears. 
1902 Experiments in the treatment of apple and pear scab during 

1901-1902. Victoria Agr. Dept. Journ. 1:525-528. 

Results of spraying experiments, and notes on varietal suscep- 
tibility. 

1902 Spraying experiments in 1901-1902 for black spot. Victoria 

Agr. Dept. Journ. 1:432. 

A preliminary report on the spraying experiments for the seasons 
1 90 1 and 1902. 

1903 Report of the Vegetable Pathologist. Black spot of apple and 

pear. Victoria Agr. Dept. Journ. 2:250-251. 

Tests of various combinations of bordeaux with other substances 
used on apple and pear. Some experiments are reported more briefly 
in Bulletin 17 of the department. 

1903 Spraying for black spot of the apple. Victoria Agr. Dept. 

Journ. 2:354-360. 

Addition of common salt slightly increased the efficiency of bordeaux, 
but McAlpine considers it not necessary. Spraying in bloom apparently 
did not prevent setting of fruit, but the author thinks it advisable to 
spray earlier. 

1904 Black spot of the apple. Victoria Agr. Dept. Bui. 17:1-32. 

Appearance of scab in Australia. Varietal susceptibility, and symp- 
toms of attack. Losses. Conditions favoring disease. Spraying 
experiments. 

1907 Experiments with black spot of apple. Victoria Agr. Dept. 
Journ. 5:362-363. 

Compared bordeaux with several proprietary sheep dips, and with 
phenyl, carbolic acid, and the like. None of the latter gave good results. 

1910 Report of the Vegetable Pathologist. Victoria Agr. Dept. 
Rept. 1907 1910:47-48. 

Experiments with lime-water bordeaux. 

McCarthy, Gerald 

1 89 1 Plant diseases and how to combat them. North Carolina 
Agr. Exp. Sta. Bui. 76:15. 
Short note on scab and means for prevention. 

M'Cormack, E. F. 

1910 Apple scab {Venturia pomi). Indiana State Entomologist. 

Ann. rept. 3:145-147. 
Popular description of disease and directions for control. 

McCready, S. B. 

191 1 Spraying for apple scab. Ontario Agr. Col. and Exp. Farm. 

Ann. rept. 36:42. 

Macoun, W. T. 

1901 Apple culture. Canadian Agr. Dept., Cent. Exp. Farm. Bui. 
37:67-68. 

Notes this fungus as being very troublesome during recent years. 



6i2 Bulletin 335 

1902 Spraying. Canadian Exp. Farms. Rept. 1901 : 109. 

Notes on spraying apples for scab. 

1903 Spraying. Canadian Exp. Farms. Rept. 1902:110-111 . 

Notes loss due to omission of late spraying. Note on bordeaux 
russeting of apples. 

Malley, C. W. 

1909 Spraying for apple scab or black spot. Cape Good Hope agr. 
journ. 35:202-211. 

Spraying experiments. At least half the fruit dropped from unsprayed 
trees. Third spraying caused some injury. Bordeaux injury found 
to occur as seriously when a small quantity of copper sulfate was used 
as when the quantity was large. A few trees were sprayed while in 
blossom and most of the blossoms were killed. 

Marchal, E. 

1907 Les principaux ma!adies du pommier. Brussels agr. bul. 23: 
56-58. 
Short description of disease and directions for treatment. 
1907 Rapport sur les observations effectives par le service patho- 
logique de l'lnstitute Agricole de l'Etat en 1906. Brussels 
agr. bul. 23:41. 

Reports occurrence of apple scab. 

Marlatt, C. L., and Orton, W. A. 

1906 The control of the codling moth and apple scab. U. S. Agr. 

Dept. Farmers' bul. 247:12-21. 
Discussion of the disease and methods of control. 

Massee, G. 

1899 Apple scab. A text-book of plant diseases, pp. 302-304, 435. 

Short botanical description of this fungus, together with preventive 
measures. 

1907 Apple scab. A text-book of plant diseases, pp. 302-304. 

Brief description of disease, giving authorized methods of control. 

Massey, W. F. 

1893 The culture of orchard and garden fruits. North Carolina 
Agr. Exp. Sta. Bul. 92:87-88. 

General information concerning scab, and list of varieties most and 
least liable to attack. 

1900 The diseases and insects affecting apple trees in North Carolina, 

with suggestions for their destruction. North Carolina 
State Bd. Agr. Bul. 21:28-39. 

Discussion of treatment for apple diseases in general. 

Maynard, S. T. 

1891 Fungicides and insecticides on the apple, pear, and plum. 
Massachusetts (Hatch) Agr. Exp. Sta. Bul. 11:12-16. 

Reports various spraying experiments, which were not very success- 
ful in preventing apple scab. 



Scab Disease of Apples 613 

Mohr, Karl 

1900 Boric-lit tiber die im sommer [899 angestellten versuche behufs 

bekampfung pflanzlicher schmarotzer auf reben und kernobst. 
Zeitsch. pflanzenkr. 10: 2 70- 274. 

Reports the use of basic calcium sulfid as a summer spray for apple 
ami pear scab and for grape mildews. 

1901 Versuche tiber die pilztotenden eigenschaften des sulfurins. 

Zeitsch. pflanzenkr. 11:98-99. 

Reports successful use of sulfurin (chemically calcium polysul 

for apple and pear seal) and also for several other fungous diseases. 

Morris, O. M., and Nicholson, J. F. 

1908 Orchard spraying. Oklahoma Agr. Exp. Sta. Bui. 76:27-28. 
Brief account of apple seal). 

Morse, W. J. 

1910 Notes on plant diseases in 1908. Maine Agr. Exp. Sta. Bui. 

164: 1-28. 

Notes on selfdxuled lime-sulfur as a substitute for bordeaux mixture 
for apple scab. Notes on late infection. 

Morse, W. J., and Lewis, C. E. 

1911 Maine apple diseases. Maine Agr. Exp. Sta. Bui. 185:352- 

355. 39°- 

Description of the disease and notes on the life history of the 
fungus. Storage infection noted. 

Munson, W. M. 

1892 Spraying experiments. Maine Agr. Exp. Sta. Ann. rept. 
1891: 110-118. 

Account of scab and results of favorable spraying experiments with 
eau celeste and ammoniacal solution of copper carbonate. 

1892 Spraying experiments. Maine Agr. Exp. Sta. Ann. rept. 
1892:92-98. 

Favorable results from spraying with eau celeste against scab. 
1894 Notes of spraying experiments. Maine Agr. Exp. Sta. Ann. 
rept. 1893:124-128. 

Favorable results from spraying with bordeaux against scab. 
1903 Experiments in orchard culture. Maine Agr. Exp. Sta. Bui. 
89:16-18. 

The use of an excess of potash as fertilizer did not ward off attacks 
of scab. 

1905 Summary of experiments in practical horticulture. Maine Agr. 

Exp. Sta. 6111.113:26-27. 

Ammoniacal copper carbonate was less satisfactory than bordeaux. 

1906 Orchard notes. Maine Agr. Exp. Sta. Bui. 128:69-71. 

Notes on and directions for spraying for scab. Notes on pink rot 
following stab. 



614 Bulletin 335 

1908 Orchard notes, 1907. Maine Agr. Exp. Sta. Bui. 155:143. 

Notes on spraying for scab, and directions for same. 

1909 Apple enemies and how to fight them. West Virginia Agr. 

Exp. Sta. Bui. 121 : 353-366. 

General directions for spraying apple trees. 

Niessl, G. von 

1881 Didymosphasria inasqualis (Cke.) Nssl. Fung. Eur., Raben- 
horst, no. 2663. 

Changes to above name from Sphcerella inccqualis Cke. 
1898 Bemerkung fiber " Venturia " inaequalis (Cooke) und verwandte 
formen. Hedw. beiblatt 37: 1-2. 

Criticises Aderhold for placing above species under genus Venturia 
because of presence of bristles around ostiolum. 

Norton, J. B. S., and Norman, A. J. 

1910 Controlling fungous diseases. Maryland Agr. Exp. Sta. Bui. 

143 : 177-187, 200. 

Compares self-boiled lime-sulfur, bordeaux, and sulfocide. Finds 
bordeaux the most satisfactory of the three. 

Norton, J. B. S., and Symons, T. B. 

1907 Control of insect pests and diseases of Maryland crops. Mary- 
land Agr. Exp. Sta. Bui. 115:176-177. 

Contains alphabetical list of cultivated crops, with notes on their 
pests and diseases including apple scab. 

Paddock, Wendell 

1901 Plant diseases of 1901. Colorado Agr. Exp. Sta. Bui. 69:9. 
Note on bordeaux injury. Serious on Ben Davis apples. 

Pammel, L. H. 

1885 Apple scab and leaf blight. Prairie farmer 57: 746. 

Gives abstract of article by Trelease. 
1891 Treatment of fungus diseases. Iowa Agr. Exp. Sta. Bui. 
13:48-49. 

Gives copper carbonate as fungicide for this disease. 

Peck, C. H. 

1873 Spilocaea Pomi Fr. New York State Cabinet Nat. Hist. 
Rept. 23:55. 

Reports scab common on apples in New York in 1869. 
1881 Fusicladium dendriticum Wallr. New York State Mus. Nat. 
Hist. Rept. 34 : 3 2-33 . 

Gives short account of above, including with it forms on pear and 
other hosts. 

Persoon, C. H. 

1822 Fumago Mali. Myc. Eur. 1:9. 

Brief description of above fungus, which may possibly be apple 
scab. 



Scab Disease of Apples 615 

Pickett, B. S. 

1908 Spraying apple orchards for insects and fungi. Illinois Agr. 
Exp. Sta. Circ. 120:1-36. 
General directions for spraying, with spray calendar. 

Piper, C. V. 

1893 Common fungous diseases and methods of prevention. Wash- 

ington Agr. Exp. Sta. Bui. 8:138. 

Writes briefly about apple scab and its prevention by spraying. 
1902 Orchard enemies in the Pacific Northwest. U. S. Agr. Dept. 
Farmers' bul. i53 : 33~34- 

Notes on distribution of apple scab in the Northwest, description, 
and directions for treatment. 

Powell, G. H. 

1894 The apple-scab. Garden and forest 7:297. 

List of varieties most and least affected by scab, as determined at 
Cornell University Horticultural Experiment Station. 

Quaintance, A. L., and Scott, W. M. 

19 1 2 The more important insect and fungous enemies of the fruit and 

foliage of the apple. U. S. Agr. Dept. Farmers' bul. 
492 : 23-26. 

Quinn, G. 

1907 Seasonable notes on some orchard pests. South Australia 
Agr. Dept. Journ. 10:14. 

Brief notes on apple scab. Directions for spraying. 

Reddick, D. 

1913 The apple scab situation. West. New York Hort. Soc. Proc. 

58:86-90. 
1913 Factors influencing successful orchard spraying. New York 
State Fruit Growers' Assoc. Proc. 12:51-54. 

Roberts, J. W. 

191 1 The dilute lime-sulphur sprays versus bordeaux mixtures for 
apple diseases. Is bordeaux to be abandoned? Indiana 
Hort. Soc. Trans. 1910:82-93. 

Control experiment. 

Roumeguere, C. 

1890 Fusicladium dendriticum forma microspcrma. Fungi sel. exs., 
no. 5592. 

Describes briefly this new form, of which a specimen is given. 

Ruggles, A. G., and Stakman, E. C. 

191 1 Orchard and garden spraying. Minnesota Agr. Exp. Sta. Bul. 
121 : 1-32. 
Brief description of scab and methods of treatment. 



616 Bulletin 335 

Saccardo, P. A. 

1881 Fusicladium dcndriticum (Wallr.) Fckl. var. minor. Fungi 
Italici, f. 782. 
Gives form on apple as variety of that on pear. 
1886 Fusicladium dendriticum (Wallr.) Fckl. Syll. fung. 4:345. 

Botanical description of above species. 
1886 Fusicladium dendriticum (Wallr.) Fckl. var. Soraueri (Thum.). 
Syll. fung. 4:346. 

Reduces von Thiimen's Napicladium Soraueri to variety of apple- 
scab fungus. 

Salmon, E. S. 

1908 Apple scab or black spot. London Bd. Agr. Journ. 15:182- 

195- 

Scab said to cause considerable damage in England. Susceptibility 
of varieties discussed. Directions for bordeaux treatment. For late 
infection ammoniacal copper carbonate should be used. Suggests 
winter washing of trees with strong copper-sulfate solution when twigs 
are infected. Notes finding scab on twigs. First time reported in 
England. 

1908 Apple scab or black spot and its treatment. Southeastern Agr. 

Col. (Wye, Kent). Journ. 17:304-315. 
Importance of the disease in England. 

1909 Black spot or apple scab. Southeastern Agr. Col. (Wye, 

Kent). Journ. 18:267-270. 
Control experiments. Twig infection. 

1910 A lime-sulphur wash for use on foliage. London Bd. Agr. 

Journ. 17:184-189. 

Used lime-sulfur wash of a density of 1.01 for hop mildew (Sphaer- 
otheca humuli), gooseberry mildew (S. mors-uvae), and apple scab. 
Results good on all, so far as tested. 

1910 Injury to foliage by bordeaux mixture. London Bd. Agr. 
Journ. 17: 103-114. 

Discusses bordeaux injury on both fruit and foliage. Notes injury 
due to previous attack by the fungus. Russeting of fruit due to weather 
conditions. Recommends spraying susceptible varieties very lightly. 
Advises that no other spray than bordeaux be used for scab. Lime- 
sulfur is unquestionably inferior to bordeaux, although some success 
has attended its use in the United States. 

Sanderson, E. D., Headlee, T. J., and Brooks, Charles 

1907 Spraying the apple orchard. New Hampshire Agr. Exp. Sta. 
Bui. 131:9-56. 

Popular descriptions of apple scab and results of spraying experi- 
ments. A net profit of eighty per cent for the first year is reported, 
as a result of spraying for scab and codling moth. 

Scalia, G. 

1901 Intorno ad una nuova forma di Fusicladium dendriticum 
(Wallr.) Fckl. Accad. Gioenia d. Catania. Boll. 70:1-5. 
1901. Abs. in Bot. centbl. 89:398. 1902. 

Fusicladium on Japanese loquat seems to have affinities with F. 
dendriticum and F. eriobothryae, and the name F. dendriticum eriobo- 
thryae-japonicae is given it. (Abstract consulted.) 



Scab Disease of Apples 617 

Schander, R. 

1909 Zur karbolineumfrage. Dcut. landw. prcsse 36:63-64. 

Reports spraying experiments, testing carbolineum which failed to 
control seal). 

Schroeter, J. 

1894 Fusicladium dendriticum Wallroth. Krypt.-fl. von Schles. 

Describes scab on apple and pear as a conidial stage of Venturia 
chlorospora. 

Schweinitz, L. D. de 

1834 Spilocsea fructigena aut Pomi Lk. Syn. F. N. A., p. 297. 

Reports above not rare on Newton Pippin. Seems to have originated 
the specific name fructigena. 

Scott, W. M. 

1906 The control of apple bitter-rot. U. S. Agr. Dept., Plant Indus. 

Bur. Bui. 93:33. 

Directions for combined treatment for scab and bitter rot. 

1908 Self-boiled lime-sulfur mixture as a promising fungicide. U. S. 

Agr. Dept., Plant Indus. Bur. Circ. 1:12. 

Reports no definite data, but favorable indications as to the control 
of apple scab by self-boiled lime-sulfur. 

1909 Lime-sulfur mixtures for the summer spraying of orchards. 

U. S. Agr. Dept., Plant Indus. Bur. Circ. 27:15-17. 

Commercial and self-boiled lime-sulfur used for control of scab, in 
comparison with bordeaux mixture. 

19 10 The substitution of lime-sulphur preparations for bordeaux 

mixture in the treatment of apple diseases. U. S. Agr. Dept., 
Plant Indus. Bur. Circ. 54:1-15. 

Results of experiments in Virginia, Michigan, and Arkansas, during 
the season of 1909. Lime-sulfur solution seemed the more promising. 

191 o The use of lime-sulphur sprays in the summer spraying of Vir- 
ginia apple orchards. Virginia Agr. Exp. Sta. Bui. 188: 
1-16. 

Lime-sulfur was as effective as bordeaux for apple scab, and caused 
no russeting of fruit as did bordeaux. 

191 1 The use of dilute lime-sulphur for the control of apple diseases. 

Illinois Hort. Soc. Trans. 44:138-145. 

Summarizes results of experiments favoring the use of lime-sulfur 
for scab. 

Scott, W. M., and Quaintance, A. L. 

1907 Spraying for apple diseases and the codling moth in the Ozarks. 

U. S. Agr. Dept. Farmers' bul. 283 : 1-42. 

Combined treatment for apple scab, bitter rot, apple blotch, and 
codling moth. Description of apple scab, and brief life history of the 
fungus. 



6i8 Bulletin 335 

Scribner, F. L. 

1885 Fungous diseases of plants. U. S. Agr. Dept. Ann. rept. 
1885:81. 

Suggests raking and burning leaves as of possible benefit in prevent- 
ing apple scab. 

1888 Apple scab. U. S. Agr. Dept. Ann. rept. 1887:341-347. 

Botanical account of scab, with reference to literature. Recommends 
winter treatment with iron sulfate and early spring spraying with 
bordeaux mixture. 

1890 The treatment of certain fungous diseases of plants. Tennessee 
Agr. Exp. Sta. Spec. bul. C:6. 
Recommends certain fungicides. 

1890 Fungus diseases of the grape and other plants, and their treat- 
ment, pp. 90-96. 

Short botanical account of the fungus and methods for combating 

it. 

1890 Apple-scab and its treatment. Orchard and garden 12:113- 
114. 
General article on the subject. 

Secretan, L. 

x 833 Spilocaea pomi. Mycographie Suisse 3 : 594. 

Selby, A. D. 

1897 Some diseases of orchard and garden fruits. Ohio Agr. Exp. 
Sta. Bul. 79: 129-132. 

Gives short description of apple scab and assigns it as the cause of 
the dropping of very young apples. 

1899 Investigations of plant diseases. Ohio Agr. Exp. Sta. Bul. 
111:95-115. 

Results of various spraying experiments made at different times by 
persons in Ohio. 

1910 A brief handbook of the diseases of cultivated plants in Ohio. 
Ohio Agr. Exp. Sta. Bul. 214:371. 

Notes on scab and on profit due to spraying. Directions for control. 
Notes on iron sulfate as a sticker for bordeaux. 

Sherman, Franklin 

1904 Spraying apples and pears. North Carolina Agr. Dept. Ent. 
circ. 6: 1-7. 

General directions for spraying apples for insects and fungous diseases. 

Smith, E. F. 

1894 Apple scab. Journ. myc. 7:373-374. 

Speaks of 1892 as an unusually bad year for scab in western New 
York and southwestern Michigan, because of wet spring. 

Smith, W. G. 

1885 Cracking of apples and pears caused by Cladosporium den- 
driticum. Gard. chron. n. s. 24:691. 

Notes that the fungus attacks petals, anthers, and pistils of flowers, 
in some cases causing abortion of the fruit. Extremely difficult to 
suggest method of control. 



Scab Disease of Apples 619 

(Aim mymous) 

1 89 1 Some of the most common fungi and insects — with preven- 
tives. New York (Geneva) Agr. Exp. Sta. Bui. 35:606- 
607. 

Describes briefly the effect of fungus and recommends preventive 
treatment by spraying. 

Sorauer, Paul 

1874 Entstehung der sogenannten rostflecken auf den fruchten des 

kernobstes. Vers. Deut. Naturf. u. Aerzte. Verhandl. 
1874:84-85- 

1875 Entstehung der sogenannten rostflecken auf den friichten des 

kernobstes. Bot. ztg. 33 : 50-5 1 . 

Botanical account of apple scab on leaves and fruit. 
1875 Die' entstehung der rostflecken, auf apfeln und birnen. Ver. 
Beford. Gart. Kgl. Preuss. Staaten. Monatsschrift 18:5-15. 

Description of Fusicladium dendriticum and disease caused by the 
fungus. The conidial stage is well illustrated. 

1882 Fusicladium dendriticum. Die obstbaumkrankheiten, pp. 
100-103. 

Account of the gross and microscopical appearance of scab on leaves 
and fruit. 
1886 Die rostflecke der apfel und birnen. Handbuch der pflanzen- 
krankheiten, p. 392. 

Describes apple scab briefly. 
1888 Fusicladium dendriticum. Die schaden der kulturpflanzen, 
p. 224. 

Short note on this fungus. 

Sprague, C. J. 

1856 Contributions to New England mycology. Boston Soc. Nat. 
Hist. Proc. 5:329. 

Lists Spiloccea fructigena from New England but does not give host. 

(Anonvmous) 

1895 Spraying pear and apple orchards in 1894. New York (Geneva) 
Agr. Exp. Sta. Bui. 84: 19-20, 31-33. 

Recommends time for spraying and notes varieties injured by bor- 
deaux mixture. 

Stene, A. E. 

191 o Some suggestions on apple growing in Rhode Island. Rhode 
Island State Bd. Agr. Rept. 25: 1 51-153, 155. 
Brief notes on scab and control methods. 

Stevens, F. L., and Sherman, Jr., Franklin 

1903 Insect and fungus enemies of the apple, pear, and quince, with 
methods of treatment. North Carolina Agr. Exp. Sta. Bui. 
183:64-66. 



620 Bulletin 335 

Stewart, F. C. 

1909 Recent investigations on plant diseases. West. New York 

Hort. Soc. Proc. 54 : 7 7-8 1 . 

Includes note on lime-sulfur solution for apple scab. 

Stewart, F. C, and Blodgett, F. H. 

1899 A fruit-disease survey of the Hudson valley in 1899. New 

York (Geneva) Agr. Exp. Sta. Bui. 167 : 283. 

But very little scab during the dry season of 1899. Notes occurrence 
of scab on twigs of the Lady apple. 

Stewart, F. C, Rolfs, F. M., and Hall, F. H. 

1900 A fruit-disease survey of western New York in 1900. New 

York (Geneva) Agr. Exp. Sta. Bui. 191 : 295. 

Stewart, J P. 

1910 The apple in Pennsylvania: varieties, planting, and general 

care. Pennsylvania Agr. Exp. Sta. Bui. 106: 11-13. 
Brief notes on apple scab and its control. 

Stinson, J. T. 

1892 Apple scab, codling moth, and plum curculio. Arkansas Agr. 
Exp. Sta. Bui. 21:57-59. 

Directions for spraying with bordeaux mixture. 
1894 Apple scab. Arkansas Agr. Exp. Sta Bui. 26:23-33, 37-44. 
Favorable results from spraying with bordeaux mixture. Notes on 
damage caused by scab over the State. 

1896 Spraying experiment. Arkansas Agr. Exp. Sta. Bui. 39:20- 
22. 

Notes that spraying experiments were not profitable because of the 
lack of scab that year. 

1900 Preliminary report on bitter rot or ripe rot of apples. Missouri 
Fruit Sta. Rept. 1900: 15-17. 

Results of spraying for scab with bordeaux. Scab was more easily 
controlled by bordeaux than was bitter rot. 

Stone, G. E. 

1907 Potato and apple scab. Massachusetts State Bd. Agr. Nature 
leaflet 7:3-4. 
Popular description of disease and directions for treatment. 

Streinz, W. M. 

1862 Nomenclator fungorum, pp. 187, 280, 30S. 

Gives Fumago mali Pers. and Helminthosporium pyrorum Tib. as 
synonyms of Cladosporium dendriticum Wallr. 

Sturgis, W. C. 

1892 Scab. Connecticut Agr. Exp. Sta. Bui. 111:3-4. 
Briefly describes scab and its prevention by spraying. 



Scab Disease of Apples 621 

1893 Scab. Connecticut Agr. Exp. Sta. 6111.115:3-4. 

Short note on apple scab. 

1894 Spraying for scab of apple and pear. Connecticut Agr. Exp. 

Sta. Ann. rept. 17:72-73. 

Results of spraying experiments including winter and summer treat- 
ments. 

1901 Scab [Fusicladium dendriticum (Wallr.) Fckl.]. Connecticut 
Agr. Exp. Sta. Ann. rept. 24:258. 
A few references to literature. 

Taft, L. R. 

1890 Report on experiments with remedies for the apple scab. U. S. 
Agr. Dept., Veg. Path. Div. Bui. 11 : 30-38. 

Experiments in spraying with different fungicides, of which modified 
eau celeste and ammoniacal copper carbonate gave the best results. 

1890 Report on the experiments made in 1889 in the treatment of 
apple scab in Michigan. Michigan Agr. Exp. Sta. Bui. 
59:30-42. 
Obtained best results from use of modified eau celeste. 
1892 Insecticides and fungicides. Michigan Agr. Exp. Sta. Bui. 
83:12-14. 

Short description of the fungus, with recommendations for spraying. 
1907 Orchard spraying. Michigan Agr. Exp. Sta. Spec. bul. 
37:25-26. 
Brief directions for spraying for scab and codling moth. 

Taft, L. R., and Davis, G. C. 

1895 The pests of the orchard and garden. Michigan Agr. Exp. 

Sta. Bul. 121 : 19-21. 

Writes briefly about scab and recommends use of bordeaux mixture. 

Taft, L. R., and Smith, C. D. 

1907 Spraying calendar. Michigan Agr. Exp. Sta. Spec. bul. 
36:266-274. 

General directions for spraying for scab. 

Taft, L. R., and Wilken, F. A. 

1909 Annual report of the South Haven sub-station for 1908. Mich- 

igan Agr. Exp. Sta. Spec. bul. 48: 16-24. 

Note on scab as affecting set of fruit, and report of use of lime-sulfur 
and calcium benzoate as summer sprays. Rex and Niagara lime- 
sulfur solutions, diluted 1-25, used on foliage of apples, pears, cherries, 
and plums. 

Tetzner, 

19 10 Die schwefelkalkbriihe und ihr'einfluss auf die kupfernen 

spritzen. Deut. obstbaume ztg. 14:179-180. 

Note suggesting use of lime-sulfur on apple trees. 



622 Bulletin 335 

Thiimen, F. von 

1875 Napicladium, eine neue hyphomyceten-gattung. Hedw. 14: 

3-4- 

Describes scab fungus on apple fruit as belonging to a new genus. 

1875 Napicladium Soraucri Thucm. Myc. uni., no. 91. 

Gives specimen and description of his new genus. 
1 89 1 Ueber einige besonders beachtenswerte durch parasitische 
pilze hervorgerufene krankheiten der apfelbaumblatter. 
Zeitsch. pflanzenkr. 1: 167-169. 

Discusses apple scab and gives Fusicladium dcndriticum var. Soraueri 
and Napicladium Soraucri as synonyms. 

Tillinghast, J. A., and Adams, G. E. 

1899 Suggestions as to spraying. Rhode Island Agr. Exp. Sta. 
Bui. 52 : 20-2 1. 
Appearance of apple scab described and remedy suggested. 

Trelease, William 

1884 The apple scab and leaf blight. Wisconsin Agr. Exp. Sta. 
Ann. rept. 1 : 45—56. 

Gives extended account of scab and of varieties of apples attacked, 
and recommends raking together and burning leaves. 

1884 Fusicladium dendriticum (Walk.). Prel. list par. fungi Wis- 
consin, p. 15. 

Lists this as very destructive and on following hosts: Pryus makes, 
P. coronaria, P. prunifolia. 

Voges, Ernst 

1907 Ueber die schorfkrankheit der obstbaume. Deut. landw. 
presse 34:276-277, 284-285, 290-291. 

Organisms described at length. Methods of entrance to host, effect 
on host, methods of propagation. Varietal susceptibility. Infection of 
twigs. 

1910 Die bekampfung des Fusicladium. Zeitsch. pflanzenkr. 20: 

385-393- 

Red apples considered resistant. Twig infection. Life history. 

Waite, M. B. 

191 o Experiments on the apple with some new and Httle-known 
fungicides. U. S. Agr. Dept., Plant Indus. Bur. Circ. 
58:1-19. 

Used iron sulfid, prepared by adding iron sulfate to self-boiled lime- 
sulfur; tested various modifications of bordeaux mixture. 

Walker, Ernest 

1906 Suggestions upon the care of apple orchards. Arkansas Agr. 
Exp. Sta. Bui. 91 : 166-168, 204. 

Dust sprays proved inefficient. Directions for spraying apple 
orchards. 



Scab Disease of Apples 623 

Wallace, Errett 

1909 Apple scab. Niagara Sprayer Company. Fellowship rept. 
2: 1 -10. 

Popular description of disease and fungus. Comparison of bordeaux 
and lime-sulfur solution for control of apple scab. 

1909 The value of lime-sulphur solutions as fungicides. Amcr. 

Pomol. Soc. Proc. 1909:112-113. 

Control experiments with lime-sulfur. 

1910 Spray injury induced by lime-sulfur preparations. Cornell 

Univ. Agr. Exp. Sta. Bui. 28S: 101-137. 

Notes on spray injury to fruit and foliage. Explanations and methods 
of avoiding such injury are suggested. Scab infection previous to the 
application of the spray is found to favor injury. 

1910 Venturia inaequalis, ascospore dissemination and infection. 

Science n. s. 31:753-754- 

Brief notes on method of ascospore discharge and infection. 

191 1 Lime-sulfur as a summer spray. Cornell Univ. Agr. Exp. Sta. 

Bui. 289: 139-162. 

Popular account of experiments in the control of apple scab. Various 
modifications of lime-sulfur preparations are compared with one another 
and with bordeaux mixture. Early attack of scab was found to be 
largely cause of poor set of fruit in 1910. 

Wallace, Errett, Blodgett, F. M., and Hesler, Lex R. 

191 1 Studies of the fungicidal value of lime-sulfur preparations. 
Cornell Univ. Agr. Exp. Sta. Bui. 290:163-207. 

Records laboratory tests, verified by field experiments, of the fungi- 
cidal value of many modifications of lime-sulfur solution and other 
fungicides. 

Wallroth, F. G. 

x 833 Cladosporium dendriticum W. Fl. crypt. Germ. 2:4:169. 

Describes this as a new species on apple, with queried reference to 
identity with Persoon's Fumago mali. 

Warren, G. F. 

1905 An apple orchard survey of Orleans county. Cornell Univ. 

Agr. Exp. Sta. Bui. 229:475-481. 

Notes on spraying. Estimate of profit due to spraying. 

1906 Spraying. New Jersey Agr. Exp. Sta. Bui. 194:12-15. 

Popular notes on spraying apples, with directions. 
1909 Elements of agriculture, p. 253. 
Brief note on apple scab. 

Warren, G. F., and Voorhees, J. A. 

1907 Report of the Horticulturist. New Jersey Agr. Exp. Sta. 

Rept. 27:225. 

Attributes cloudiness on pears to an attack of the apple-scab fungus 
{Venturia incequalis). 



624 Bulletin 335 

Watkins, O. S. 

191 1 Summary of various spraying experiments for the summer of 
1910. Illinois Hort. Soc. Trans. 44: 145-169. 

Bordeaux mixture considered more effective than lime-sulfur. 

Weed, C. M. 

1890 Apple scab. Ohio Agr. Exp. Sta. Bui. 14:188. 

Reports injury to fruit from spraying with bordeaux mixture. 

Whetzel, H. H., and Stewart, F. C. 

1908 The control of plant diseases. Cornell Univ. Agr. Exp. Sta. 
Bui. 252 : 190. 

Brief directions for controlling scab. 

Whitten, J. C. 

1895 Spraying orchards and vineyards. Missouri Agr. Exp. Sta. 
Bui. 31 : 1-19. 

Favorable results from spraying with bordeaux mixture. 

Wilcox, E. M. 

1905 Diseases of the apple, cherry, peach, pear, and plum, with 
methods of treatment. Alabama Agr. Exp. Sta. Bui. 
132 :gj-io2. 
Brief description, with directions for control. 

Williams, T. A. 

1893 Common fungous and insect foes of farm and garden. South 
Dakota Agr. Exp. Sta. Bui. 35:79. 
Gives short note on apple scab. 

Winter, Georg 

1875 Ucber Napicladium soraueri Thumen. Hedw. 14:35-36. 

Shows that above fungus is not distinct from Fusicladium dendriticum 

(Wallr.). 

1880 Venturia inaequalis Wint. Myc. uni., von Thumen, no. 1544. 
Gives this name for first time to specimens on apple leaves. 



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